Canvas2d: Implement rerouting event by hit region's control.

third_party/WebKit/Source/modules/canvas2d/MouseEventHitRegion.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "modules/canvas2d/MouseEventHitRegion.h"
 
 #include "core/events/MouseEvent.h"
-#include "core/html/HTMLCanvasElement.h"
 
 namespace blink {
 
 String MouseEventHitRegion::region(MouseEvent& event)
 {
-    if (!event.target() || !isHTMLCanvasElement(event.target()->toNode()))
-        return String();
-
-    HTMLCanvasElement* canvas = toHTMLCanvasElement(event.target()->toNode());
-    return regionIdFromAbsoluteLocation(*canvas, event.absoluteLocation());
+    return event.region();
 }
 
 } // namespace blink