Add ComposedTreeTraversal::lastWithin/lastWithinOrSelf

Source/core/dom/shadow/ComposedTreeTraversal.h

Index: Source/core/dom/shadow/ComposedTreeTraversal.h
diff --git a/Source/core/dom/shadow/ComposedTreeTraversal.h b/Source/core/dom/shadow/ComposedTreeTraversal.h
index 9b24415fbfb47d224b2aded7bf28bfa0059daf1e..32abde58a665a11617813fd6110a360b8ea06b08 100644
--- a/Source/core/dom/shadow/ComposedTreeTraversal.h
+++ b/Source/core/dom/shadow/ComposedTreeTraversal.h
@@ -95,6 +95,9 @@ public:
     // composed tree.
     static unsigned countChildren(const Node&);
 
+    static Node* lastWithin(const Node&);
+    static Node& lastWithinOrSelf(Node&);

[hayato, 2015/03/26 03:28:27]

Input |Node&| can't be |const Node&|?

I guess our concern is the case where it returns self, however, 
I think it's okay that |const Node&| will be discarded for self in this case in
practical.

[hajimehoshi, 2015/03/26 03:37:18]

As you mentioned, I was worried the case when this function returns the argument
itself. You mean I can use const_cast in this case? I'll try to do that.

[hajimehoshi, 2015/03/26 03:43:26]

Done.

+
 private:
     enum TraversalDirection {
         TraversalDirectionForward,