Implement basic parts of hit regions on canvas2d.

Source/core/accessibility/AXNodeObject.cpp

Index: Source/core/accessibility/AXNodeObject.cpp
diff --git a/Source/core/accessibility/AXNodeObject.cpp b/Source/core/accessibility/AXNodeObject.cpp
index 6410e9cafdbb12241ee581154b6f6e2862e2affa..a660c567575d811404203fe6185ffd529634e7a9 100644
--- a/Source/core/accessibility/AXNodeObject.cpp
+++ b/Source/core/accessibility/AXNodeObject.cpp
@@ -1247,6 +1247,23 @@ LayoutRect AXNodeObject::elementRect() const
     if (!m_explicitElementRect.isEmpty())
         return m_explicitElementRect;
 
+    if (node()->parentElement()->isInCanvasSubtree()) {

[dmazzoni, 2014/06/09 07:27:26]

OK for now, but add a FIXME indicating that this is inefficient if there are a
lot of elements in the canvas.

AXComputedObjectAttributeCache might be a good candidate for speeding this up.
Again, don't worry about fixing in this change but please make a note of it.

[zino, 2014/06/10 10:09:34]

Done.

+        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());
+                }
+            }
+        }
+
+        return rect;

[dmazzoni, 2014/06/09 07:27:26]

If the resulting rect is still empty, fall through to the code below - a rect
somewhere in the canvas is much better than an empty rect.

[zino, 2014/06/10 10:09:34]

Done.

+    }
+
     // AXNodeObjects have no mechanism yet to return a size or position.

[dmazzoni, 2014/06/09 07:27:26]

Delete this first bit of the comment and change it to say, if this object
doesn't have an explicit element rect or one computable from its children,
return the position of the ancestor that does have...

[zino, 2014/06/10 10:09:34]

Done.

     // 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.