[DevTools] Cleanup DOMExtension.js.

third_party/WebKit/LayoutTests/http/tests/inspector/layers-test.js

 function initialize_LayerTreeTests()
 {
     InspectorTest.layerTreeModel = function()
     {
         if (!InspectorTest._layerTreeModel)
             InspectorTest._layerTreeModel = WebInspector.LayerTreeModel.fromTarget(InspectorTest.mainTarget);
         return InspectorTest._layerTreeModel;
     }
 
     InspectorTest.labelForLayer = function(layer)
@@ skipping 87 matching lines @@
                 InspectorTest.addObject(layer._scrollRects);
         }
 
         InspectorTest.addResult("Model elements dump");
         InspectorTest.layerTreeModel().layerTree().forEachLayer(dumpScrollRectsForLayer.bind(this));
     }
 
     InspectorTest.dispatchMouseEvent = function(eventType, button, element, offsetX, offsetY)
     {
         var totalOffset = element.totalOffset();
-        var scrollOffset = element.scrollOffset();
         var eventArguments = {
             bubbles: true,
             cancelable: true,
             view: window,
-            screenX: totalOffset.left - scrollOffset.left + offsetX,
-            screenY: totalOffset.top - scrollOffset.top + offsetY,
+            screenX: totalOffset.left - element.scrollLeft + offsetX,
+            screenY: totalOffset.top - element.scrollTop + offsetY,
             clientX: totalOffset.left + offsetX,
             clientY: totalOffset.top + offsetY,
             button: button
         };
         if (eventType === "mouseout") {
             eventArguments.screenX = 0;
             eventArguments.screenY = 0;
             eventArguments.clientX = 0;
             eventArguments.clientY = 0;
         }
         element.dispatchEvent(new MouseEvent(eventType, eventArguments));
     }
 }