Implement basic parts of hit regions on canvas2d.

Source/core/accessibility/AXNodeObject.cpp

 /*
 * Copyright (C) 2012, Google Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1.  Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 1229 matching lines @@
 
     return String();
 }
 
 LayoutRect AXNodeObject::elementRect() const
 {
     // First check if it has a custom rect, for example if this element is tied to a canvas path.
     if (!m_explicitElementRect.isEmpty())
         return m_explicitElementRect;
 
-    // AXNodeObjects have no mechanism yet to return a size or position.
-    // For now, let's return the position of the ancestor that does have a position,
+    // FIXME: If there are a lot of elements in the canvas, it will be inefficient.
+    // We can avoid the inefficient calculations by using AXComputedObjectAttributeCache.
+    if (node()->parentElement()->isInCanvasSubtree()) {
+        LayoutRect rect;
+
+        for (Node* child = node()->firstChild(); child; child = child->nextSibling()) {
+            if (child->isHTMLElement()) {
+                if (AXObject* obj = axObjectCache()->get(child)) {
+                    if (rect.isEmpty())
+                        rect = obj->elementRect();
+                    else
+                        rect.unite(obj->elementRect());
+                }
+            }
+        }
+
+        if (!rect.isEmpty())
+            return rect;
+    }
+
+    // If this object doesn't have an explicit element rect or computable from its children,
+    // for now, let's return the position of the ancestor that does have a position,
     // and make it the width of that parent, and about the height of a line of text, so that it's clear the object is a child of the parent.
 
     LayoutRect boundingBox;
 
     for (AXObject* positionProvider = parentObject(); positionProvider; positionProvider = positionProvider->parentObject()) {
         if (positionProvider->isAXRenderObject()) {
             LayoutRect parentRect = positionProvider->elementRect();
             boundingBox.setSize(LayoutSize(parentRect.width(), LayoutUnit(std::min(10.0f, parentRect.height().toFloat()))));
             boundingBox.setLocation(parentRect.location());
             break;
@@ skipping 419 matching lines @@
     float range = maxValueForRange() - minValueForRange();
     float value = valueForRange();
 
     value += range * (percentChange / 100);
     setValue(String::number(value));
 
     axObjectCache()->postNotification(node(), AXObjectCache::AXValueChanged, true);
 }
 
 } // namespace WebCore