Make pack and unpack image extensions enableable.

BUG=angleproject:1523

Change-Id: Ic728a777ad7e05373de03ee98e9b0a17101cd45d
Reviewed-on: https://chromium-review.googlesource.com/688102
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 a254961..f932954 100644
--- a/src/libANGLE/validationES2.cpp
+++ b/src/libANGLE/validationES2.cpp
@@ -5374,6 +5374,12 @@
             break;
 
         case GL_PACK_REVERSE_ROW_ORDER_ANGLE:
+            if (!context->getExtensions().packReverseRowOrder)
+            {
+                ANGLE_VALIDATION_ERR(context, InvalidEnum(), EnumNotSupported);
+            }
+            break;
+
         case GL_UNPACK_ROW_LENGTH:
         case GL_UNPACK_IMAGE_HEIGHT:
         case GL_UNPACK_SKIP_IMAGES: