Vulkan: Add glslang validator to build.

This binary will be used to generate internal shaders for
clearing/blit/etc.

Bug: angleproject:2339
Change-Id: I13db7d7a4f5a0155798c9b9a6d2972dd69a680ef
Reviewed-on: https://chromium-review.googlesource.com/904683
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>

third_party/glslang/BUILD.gn

@@ -134,3 +134,15 @@ static_library("glslang") {
     sources += [ "src/glslang/OSDependent/Unix/ossource.cpp" ]
   }
 }
+
+executable("glslang_validator") {
+  sources = [
+    "src/StandAlone/DirStackFileIncluder.h",
+    "src/StandAlone/ResourceLimits.h",
+    "src/StandAlone/StandAlone.cpp",
+    "src/StandAlone/Worklist.h",
+  ]
+  public_deps = [
+    ":glslang",
+  ]
+}