Implement textureGrad.

BUG=angle:564

Change-Id: I95dcd95a274f6d560250c197be0d6e64a5b23516
Reviewed-on: https://chromium-review.googlesource.com/187081
Tested-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/compiler/translator/OutputHLSL.h b/src/compiler/translator/OutputHLSL.h
index e8d24f8..79e5572 100644
--- a/src/compiler/translator/OutputHLSL.h
+++ b/src/compiler/translator/OutputHLSL.h
@@ -105,7 +105,8 @@
             LOD,
             LOD0,
             SIZE,   // textureSize()
-            FETCH
+            FETCH,
+            GRAD
         };
 
         TBasicType sampler;