Implement basic parts of hit regions on canvas2d.

Source/core/html/canvas/MouseEventHitRegion.idl

Index: Source/core/html/canvas/MouseEventHitRegion.idl
diff --git a/Source/core/html/canvas/MouseEventHitRegion.idl b/Source/core/html/canvas/MouseEventHitRegion.idl
new file mode 100644
index 0000000000000000000000000000000000000000..e501b4ee24513cd3af697f0cb939c95fc7f02281
--- /dev/null
+++ b/Source/core/html/canvas/MouseEventHitRegion.idl
@@ -0,0 +1,11 @@
+// Copyright 2014 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.
+
+// http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#MouseEvent-partial
+
+[
+    RuntimeEnabled=ExperimentalCanvasFeatures
+] partial interface MouseEvent {
+    readonly attribute DOMString? region;
+};