Support EXT_multisample_compatibility in the GL backend

BUG=angleproject:1377

Change-Id: Ie14aceca8e01f1cbc93fd5bd06d986336fb752b3
Reviewed-on: https://chromium-review.googlesource.com/343501
Reviewed-by: Sami Väisänen <svaisanen@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Sami Väisänen <svaisanen@nvidia.com>
diff --git a/src/libANGLE/validationES.cpp b/src/libANGLE/validationES.cpp
index 40db7aa..76e8c64 100644
--- a/src/libANGLE/validationES.cpp
+++ b/src/libANGLE/validationES.cpp
@@ -102,6 +102,11 @@
 {
     switch (cap)
     {
+      // EXT_multisample_compatibility
+      case GL_MULTISAMPLE_EXT:
+      case GL_SAMPLE_ALPHA_TO_ONE_EXT:
+          return context->getExtensions().multisampleCompatibility;
+
       case GL_CULL_FACE:
       case GL_POLYGON_OFFSET_FILL:
       case GL_SAMPLE_ALPHA_TO_COVERAGE: