Bug 1515647 - Fix bustages on PContentChild.h in non-Nightly builds; r=asuth

dom/ipc/ContentChild.h

@@ -728,6 +728,8 @@ class ContentChild final : public PContentChild,
 
 #ifdef NIGHTLY_BUILD
   virtual PContentChild::Result OnMessageReceived(const Message& aMsg) override;
+#else
+  using PContentChild::OnMessageReceived;
 #endif
 
   virtual PContentChild::Result OnMessageReceived(const Message& aMsg,