Add new override required by glslang's BUILD.gn

Also rolls glslang.

257e9a3f19..f5ed7a69d5

2020-07-03 marcin.slusarz Add --quiet option.
2020-07-05 ShabbyX gn: Fix dawn tests in Chromium
2020-07-05 ShabbyX gn: Fix `gn gen --check` by adding missing dependency
2020-07-03 ShabbyX gn: Optionally disable optimizations and HLSL

Bug: angleproject:4814
Change-Id: Ifcffe72445a315798bf7414b236869055cef5bf0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2282947
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>

DEPS

@@ -25,7 +25,7 @@ vars = {
   'vk_gl_cts_revision': '44ef9e08c4592207c0f3d81ed385bac078f1925a',
 
   # Current revision of glslang, the Khronos SPIRV compiler.
-  'glslang_revision': '4728509962e673ed06fefc27632baeca20f1158a',
+  'glslang_revision': 'f5ed7a69d5d64bd3ac802712c24995c6c12d23f8',
 
   # Current revision of googletest.
   # Note: this dep cannot be auto-rolled b/c of nesting.

build_overrides/glslang.gni

@@ -4,3 +4,6 @@
 
 # Paths to glslang dependencies in ANGLE
 glslang_spirv_tools_dir = "//third_party/spirv-tools/src"
+
+# Temporary to unblock glslang roll.  Will change to true afterwards.
+glslang_angle = false