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

third_party/WebKit/Source/core/events/MouseEvent.idl

 /*
  * Copyright (C) 2006, 2007 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 52 matching lines @@
     [MeasureAs=MouseEventX] readonly attribute long x;
     [MeasureAs=MouseEventY] readonly attribute long y;
     [MeasureAs=MouseEventOffsetX] readonly attribute long offsetX;
     [MeasureAs=MouseEventOffsetY] readonly attribute long offsetY;
 
     // Pointer Lock
     // https://dvcs.w3.org/hg/pointerlock/raw-file/default/index.html#extensions-to-the-mouseevent-interface
     [MeasureAs=MouseEventMovementX] readonly attribute long movementX;
     [MeasureAs=MouseEventMovementY] readonly attribute long movementY;
 
+    // Canvas Hit Regions
+    // https://html.spec.whatwg.org/multipage/scripting.html#MouseEvent-partial
+    [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute DOMString? region;
+
     // Non-standard
     [MeasureAs=MouseEventFromElement] readonly attribute Node fromElement;
     [MeasureAs=MouseEventToElement] readonly attribute Node toElement;
     [MeasureAs=MouseEventWhich] readonly attribute long which;
     [Measure] readonly attribute long layerX;
     [Measure] readonly attribute long layerY;
 };