Rename treeRoot to rootNode

As a result of the discussion below, it has been decided that Node.treeRoot should be renamed to Node.rootNode.
https://github.com/w3c/webcomponents/issues/80

See the DOM standard link here : https://dom.spec.whatwg.org/#dom-node-rootnode
BUG=531990

Review URL: https://codereview.chromium.org/1771233002

Cr-Commit-Position: refs/heads/master@{#380042}
6 files changed