Add standalone glslang_validator as a download hook.

This change allows us to download a copy of the standalone glslang
validator during gclient runhooks. This decouples run_code_generation
from the version of glslang stored in source tree. This prevents the
error where a developer would run into a code generation conflict due
to a change in version of glslang.

Currently only the Windows version is stored in the tree. A follow-up
CL will add Linux. The validator will only initially support Windows
or Linux.

Documentation for the script is located in tools/glslang/README.md.
This CL also updates the Vulkan shader generation script to use the
new binary for Windows.

Bug: angleproject:3333
Change-Id: Ia6146a283a2bcfdf84c42411d6b5d8bae136bf1b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1553823
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
diff --git a/DEPS b/DEPS
index 0086897..a8f38be 100644
--- a/DEPS
+++ b/DEPS
@@ -295,6 +295,20 @@
       '{angle_root}/build/fuchsia/update_sdk.py',
     ],
   },
+
+  # Download glslang validator binary for Windows.
+  {
+    'name': 'win_glslang_validator',
+    'pattern': '.',
+    'condition': 'checkout_win and not build_with_chromium',
+    'action': [ 'download_from_google_storage',
+                '--no_resume',
+                '--platform=win32*',
+                '--no_auth',
+                '--bucket', 'angle-glslang-validator',
+                '-s', '{angle_root}/tools/glslang/glslang_validator.exe.sha1',
+    ],
+  },
 ]
 
 recursedeps = [