Implement basic parts of hit regions on canvas2d.

Source/core/html/canvas/MouseEventHitRegion.h

+// 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.
+
+#ifndef MouseEventHitRegion_h
+#define MouseEventHitRegion_h
+
+#include "core/events/MouseEvent.h"
+#include "core/html/HTMLCanvasElement.h"
+#include "core/html/canvas/CanvasRenderingContext.h"
+#include "core/html/canvas/CanvasRenderingContext2D.h"
+
+namespace WebCore {
+
+class MouseEventHitRegion {
+public:
+    static String region(MouseEvent& event, bool& isNull)
+    {
+        if (!isHTMLCanvasElement(event.target()->toNode())) {
+            isNull = true;
+            return String();
+        }
+
+        HTMLCanvasElement* canvas = toHTMLCanvasElement(event.target()->toNode());
+        CanvasRenderingContext* context = canvas->renderingContext();
+        if (!context || !context->is2d()) {
+            isNull = true;
+            return String();
+        }
+
+        HitRegion* hitRegion = toCanvasRenderingContext2D(context)->
+            hitRegionAtPoint(LayoutPoint(event.offsetX(), event.offsetY()));
+
+        String id;
+        if (hitRegion)
+            id = hitRegion->id();
+
+        isNull = id.isEmpty();
+
+        return id;
+    }
+};
+
+} // namespace WebCore
+
+#endif