Re-land "Vulkan: Add glslang validator to build."

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

Re-land fixes missing GN dependencies.

Bug: angleproject:2339
Change-Id: I99a966906f6cd31049807b7b7adf39484c951622
Reviewed-on: https://chromium-review.googlesource.com/924177
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>

third_party/glslang/BUILD.gn

@@ -132,3 +132,18 @@ static_library("glslang") {
     sources += [ "src/glslang/OSDependent/Unix/ossource.cpp" ]
   }
 }
+
+executable("glslang_validator") {
+  sources = [
+    "src/StandAlone/DirStackFileIncluder.h",
+    "src/StandAlone/StandAlone.cpp",
+  ]
+  configs -= [ "//build/config/compiler:chromium_code" ]
+  configs += [ "//build/config/compiler:no_chromium_code" ]
+  deps = [
+    "//build/config:exe_and_shlib_deps",
+  ]
+  public_deps = [
+    ":glslang",
+  ]
+}