Add WebGL validation extensions to ANGLE.

BUG=angleproject:1523

Change-Id: I6fecb5055ed8087665aeee34b3a066ea8f38d51b
Reviewed-on: https://chromium-review.googlesource.com/386281
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/validationES.cpp b/src/libANGLE/validationES.cpp
index 339624c..0846f0d 100644
--- a/src/libANGLE/validationES.cpp
+++ b/src/libANGLE/validationES.cpp
@@ -482,7 +482,8 @@
               break;
 
           case GL_DEPTH_STENCIL_ATTACHMENT:
-              if (context->getClientMajorVersion() < 3)
+              if (!context->getExtensions().webglCompatibility &&
+                  context->getClientMajorVersion() < 3)
               {
                   context->handleError(Error(GL_INVALID_ENUM));
                   return false;