Bug 1181176 - Pass along x and y position for context menus on Graphene. r=fabrice

dom/browser-element/BrowserElementChildPreload.js

@@ -868,6 +868,10 @@ BrowserElementChild.prototype = {
       }
     }
 
+    // Pass along the position where the context menu should be located
+    menuData.clientX = e.clientX;
+    menuData.clientY = e.clientY;
+
     // The value returned by the contextmenu sync call is true if the embedder
     // called preventDefault() on its contextmenu event.
     //