Add a GN argument to enable/disable commit_id.

BUILD.gn

@@ -32,6 +32,9 @@ declare_args() {
   # Don't build extra (test, samples etc) for Windows UWP. We don't have
   # infrastructure (e.g. windowing helper functions) in place to run them.
   angle_build_all = !build_with_chromium && !angle_is_winuwp && angle_has_build
+
+  # Enable generating current commit information using git
+  angle_enable_commit_id = true
 }
 
 if (angle_build_all) {
@@ -465,7 +468,7 @@ action("commit_id") {
   # Add git as a dependency if it is available.
   angle_git_is_present =
       exec_script("src/commit_id.py", [ "check" ], "value") == 1
-  if (angle_git_is_present) {
+  if (angle_enable_commit_id && angle_git_is_present) {
     # commit id should depend on angle's HEAD revision
     inputs = [ ".git/HEAD" ]
   }