Add enough ES 3.1 enums to support initializing the dEQP ES 3.1 tests.

BUG=angleproject:1442

Change-Id: Iece3efb272fdcbe004d4136129ba7bfe74ba5265
Reviewed-on: https://chromium-review.googlesource.com/405530
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/validationES2.cpp b/src/libANGLE/validationES2.cpp
index 1d7b894..94679b8 100644
--- a/src/libANGLE/validationES2.cpp
+++ b/src/libANGLE/validationES2.cpp
@@ -2288,6 +2288,16 @@
                 return false;
             }
             break;
+
+        case GL_TEXTURE_2D_MULTISAMPLE:
+            if (context->getClientVersion() < Version(3, 1))
+            {
+                context->handleError(Error(GL_INVALID_ENUM, "Context does not support GLES3.1"));
+                return false;
+            }
+            UNIMPLEMENTED();
+            break;
+
         case GL_TEXTURE_EXTERNAL_OES:
             if (!context->getExtensions().eglImageExternal &&
                 !context->getExtensions().eglStreamConsumerExternal)