Add "root" variable to DEPS.

DEPS

 vars = {
   'android_git': 'https://android.googlesource.com',
   'chromium_git': 'https://chromium.googlesource.com',
+
+  # This variable is set on the Chrome infra for compatiblity with gclient.
+  'root': '.',
 }
 
 deps = {
 
-  'build':
+  '{root}/build':
     Var('chromium_git') + '/chromium/src/build.git' + '@' + '2f3b6e8ce9e783b2a09496d70eef2974506a41c8',
 
-  'buildtools':
+  '{root}/buildtools':
     Var('chromium_git') + '/chromium/buildtools.git' + '@' + '461b345a815c1c745ac0534a6a4bd52d123abe68',
 
-  'testing':
+  '{root}/testing':
     Var('chromium_git') + '/chromium/src/testing' + '@' + '6dfa36ab2e5143fa2f7353e3af5d2935af2e61f7',
 
   # Cherry is a dEQP management GUI written in Go. We use it for viewing test results.
-  'third_party/cherry':
+  '{root}/third_party/cherry':
     Var('android_git') + '/platform/external/cherry' + '@' + '4f8fb08d33ca5ff05a1c638f04c85bbb8d8b52cc',
 
-  'third_party/deqp/src':
+  '{root}/third_party/deqp/src':
     Var('android_git') + '/platform/external/deqp' + '@' + '5bc346ba2d5465a2e6094e254f12b1586fd0097f',
 
-  'third_party/glslang/src':
+  '{root}/third_party/glslang/src':
     Var('android_git') + '/platform/external/shaderc/glslang' + '@' + '2edde6665d9a56ead5ea0e55b4e64d9a803e6164',
 
-  'third_party/googletest/src':
+  '{root}/third_party/googletest/src':
     Var('chromium_git') + '/external/github.com/google/googletest.git' + '@' + 'd175c8bf823e709d570772b038757fadf63bc632',
 
-  'third_party/libpng/src':
+  '{root}/third_party/libpng/src':
     Var('android_git') + '/platform/external/libpng' + '@' + '094e181e79a3d6c23fd005679025058b7df1ad6c',
 
-  'third_party/spirv-headers/src':
+  '{root}/third_party/spirv-headers/src':
     Var('android_git') + '/platform/external/shaderc/spirv-headers' + '@' + '98b01515724c428d0f0a5d01deffcce0f5f5e61c',
 
-  'third_party/spirv-tools/src':
+  '{root}/third_party/spirv-tools/src':
     Var('android_git') + '/platform/external/shaderc/spirv-tools' + '@' + '9996173f363729b3a97309685dbd4d78547a63a7',
 
-  'third_party/vulkan-validation-layers/src':
+  '{root}/third_party/vulkan-validation-layers/src':
     Var('android_git') + '/platform/external/vulkan-validation-layers' + '@' + '354ad3ba8b88136b82b712acab9b8fcb981beaff',
 
-  'third_party/zlib':
+  '{root}/third_party/zlib':
     Var('chromium_git') + '/chromium/src/third_party/zlib' + '@' + '24ab14872e8e068ba08cc31cc3d43bcc6d5cb832',
 
-  'tools/clang':
+  '{root}/tools/clang':
     Var('chromium_git') + '/chromium/src/tools/clang.git' + '@' + 'e70074db10b27867e6c873adc3ac7e5f9ee0ff6e',
 
-  'tools/gyp':
+  '{root}/tools/gyp':
     Var('chromium_git') + '/external/gyp' + '@' + '5e2b3ddde7cda5eb6bc09a5546a76b00e49d888f',
 }
 
@@ -60,7 +63,7 @@ hooks = [
                 '--platform=win32',
                 '--no_auth',
                 '--bucket', 'chromium-clang-format',
-                '-s', 'buildtools/win/clang-format.exe.sha1',
+                '-s', '{root}/buildtools/win/clang-format.exe.sha1',
     ],
   },
   {
@@ -72,7 +75,7 @@ hooks = [
                 '--platform=darwin',
                 '--no_auth',
                 '--bucket', 'chromium-clang-format',
-                '-s', 'buildtools/mac/clang-format.sha1',
+                '-s', '{root}/buildtools/mac/clang-format.sha1',
     ],
   },
   {
@@ -84,7 +87,7 @@ hooks = [
                 '--platform=linux*',
                 '--no_auth',
                 '--bucket', 'chromium-clang-format',
-                '-s', 'buildtools/linux64/clang-format.sha1',
+                '-s', '{root}/buildtools/linux64/clang-format.sha1',
     ],
   },
   # Pull GN binaries using checked-in hashes.
@@ -97,7 +100,7 @@ hooks = [
                 '--platform=win32',
                 '--no_auth',
                 '--bucket', 'chromium-gn',
-                '-s', 'buildtools/win/gn.exe.sha1',
+                '-s', '{root}/buildtools/win/gn.exe.sha1',
     ],
   },
   {
@@ -109,7 +112,7 @@ hooks = [
                 '--platform=darwin',
                 '--no_auth',
                 '--bucket', 'chromium-gn',
-                '-s', 'buildtools/mac/gn.sha1',
+                '-s', '{root}/buildtools/mac/gn.sha1',
     ],
   },
   {
@@ -121,14 +124,23 @@ hooks = [
                 '--platform=linux*',
                 '--no_auth',
                 '--bucket', 'chromium-gn',
-                '-s', 'buildtools/linux64/gn.sha1',
+                '-s', '{root}/buildtools/linux64/gn.sha1',
     ],
   },
+
+  {
+    # Update the Windows toolchain if necessary.  Must run before 'clang' below.
+    'name': 'win_toolchain',
+    'pattern': '.',
+    'condition': 'checkout_win',
+    'action': ['python', '{root}/build/vs_toolchain.py', 'update', '--force'],
+  },
+
   {
     # Note: On Win, this should run after win_toolchain, as it may use it.
     'name': 'clang',
     'pattern': '.',
-    'action': ['python', 'tools/clang/scripts/update.py', '--if-needed'],
+    'action': ['python', '{root}/tools/clang/scripts/update.py', '--if-needed'],
   },
 
   # Pull rc binaries using checked-in hashes.
@@ -140,18 +152,18 @@ hooks = [
                 '--no_resume',
                 '--no_auth',
                 '--bucket', 'chromium-browser-clang/rc',
-                '-s', 'build/toolchain/win/rc/win/rc.exe.sha1',
+                '-s', '{root}/build/toolchain/win/rc/win/rc.exe.sha1',
     ],
   },
 
   {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     'pattern': '.',
-    'action': ['python', 'gyp/gyp_angle'],
+    'action': ['python', '{root}/gyp/gyp_angle'],
   },
 ]
 
 recursedeps = [
   # buildtools provides clang_format.
-  'buildtools',
+  '{root}/buildtools',
 ]