Bug 593744 - nsWindow.cpp should assert if mLayerManager is null before Returning from nsWindow::GetLayerManager(). r=joedrew

--HG--
extra : rebase_source : f11c29151442b27ed8c08122d95bb37221b343f8

widget/src/windows/nsWindow.cpp

@@ -3299,6 +3299,8 @@ nsWindow::GetLayerManager(PLayersChild* aShadowManager,
       mLayerManager = CreateBasicLayerManager();
   }
 
+  NS_ASSERTION(mLayerManager, "Couldn't provide a valid layer manager.");
+
   return mLayerManager;
 }