Bug 1307372 - Avoid triggering mCallback read barrier in CallbackObject::CallbackKnownNotGray r=mccr8

dom/bindings/CallbackObject.h

@@ -126,7 +126,7 @@ public:
    */
   JS::Handle<JSObject*> CallbackKnownNotGray() const
   {
-    MOZ_ASSERT(!JS::ObjectIsMarkedGray(mCallback.get()));
+    MOZ_ASSERT(!JS::ObjectIsMarkedGray(mCallback));
     return CallbackPreserveColor();
   }