Change dEQP ES 3.1 expectations from FAIL to SKIP.

UNIMPLEMENTED debug spam was causing the tests time time out.  We can mark them
as FAIL again once the dEQP test setup/tear down code doesn't emmit so many
messages.

Implement a couple validation cases for ES 3.1 to greatly reduce the spam.

BUG=angleproject:1647
BUG=angleproject:1442

Change-Id: Ie7b4ac8737a2df1c0ada6ad53154ddf2f37d9c3c
Reviewed-on: https://chromium-review.googlesource.com/415520
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/State.cpp b/src/libANGLE/State.cpp
index 208c50b..0e07400 100644
--- a/src/libANGLE/State.cpp
+++ b/src/libANGLE/State.cpp
@@ -637,7 +637,11 @@
       case GL_PRIMITIVE_RESTART_FIXED_INDEX: setPrimitiveRestart(enabled);      break;
       case GL_RASTERIZER_DISCARD:            setRasterizerDiscard(enabled);     break;
       case GL_SAMPLE_MASK:
-          UNIMPLEMENTED();
+          if (enabled)
+          {
+              // Enabling this feature is not implemented yet.
+              UNIMPLEMENTED();
+          }
           break;
       case GL_DEBUG_OUTPUT_SYNCHRONOUS:
           mDebug.setOutputSynchronous(enabled);