Add Node.treeRoot

Add Node.treeRoot
https://bugs.webkit.org/show_bug.cgi?id=153537

Reviewed by Antti Koivisto.

Source/WebCore:

Exposed highestAncestor as Node.prototype.treeRoot, which was added to shadow DOM spec in
https://github.com/w3c/webcomponents/commit/6864a40fe4efa8a737e78512e3c85319ddc5bf8b

See also:
http://w3c.github.io/webcomponents/spec/shadow/#extensions-to-node-interface

Test: fast/shadow-dom/Node-interface-treeRoot.html

* dom/Node.idl:

LayoutTests:

Added a testharness.js test for Node.treeRoot. Also rebaselined a test.

* js/dom/dom-static-property-for-in-iteration-expected.txt: Rebaselined.
* fast/shadow-dom/Node-interface-treeRoot-expected.txt: Added.
* fast/shadow-dom/Node-interface-treeRoot.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@195682 268f45cc-cd09-0410-ab3c-d52691b4dbfc