Build bustage from bug 596451 part I - always return a value from PluginInstanceChild::ReadbackDifferenceRect.

dom/plugins/PluginInstanceChild.cpp

@@ -2617,6 +2617,8 @@ PluginInstanceChild::ReadbackDifferenceRect(const nsIntRect& rect)
         ctx->Rectangle(GfxFromNsRect(*r));
     }
     ctx->Fill();
+
+    return true;
 }
 
 void