ES31: Add atomic counter for GLSL parsing

This makes shader compiler support the new basic type 'atomic_uint'
and validate its layout qualifiers properly.

BUG=angleproject:1729
TEST=angle_unittests:AtomicCounterTest
     angle_deqp_gles31_tests:dEQP-GLES31.functional.atomic_counter.layout.invalid*
     angle_deqp_gles31_tests:dEQP-GLES31.functional.debug.negative_coverage.*.atomic*

Change-Id: Ia237eadf6ea72314f436a0abbb93a05598e71eba
Reviewed-on: https://chromium-review.googlesource.com/500088
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/compiler/translator/util.cpp b/src/compiler/translator/util.cpp
index c87b725..77ee6d2 100644
--- a/src/compiler/translator/util.cpp
+++ b/src/compiler/translator/util.cpp
@@ -392,6 +392,8 @@
             return GL_INT_IMAGE_CUBE;
         case EbtUImageCube:
             return GL_UNSIGNED_INT_IMAGE_CUBE;
+        case EbtAtomicCounter:
+            return GL_UNSIGNED_INT_ATOMIC_COUNTER;
         default:
             UNREACHABLE();
     }