This moves the SPIRV-Tools and glslang and Validation layers build
files into the root of the dep folder.

Requires an upstream Chrome change to allow for a different path
to execute scrips in GN.

Also removes the vulkan_support subdirectory.

Bug: angleproject:2339
Change-Id: I457b5b05547c092a1e7fef6dd0aaadd8068e2143
Reviewed-on: https://chromium-review.googlesource.com/900096
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>