Update DOM code to use the new Traversal<HTML*Element> API

Update DOM code to use the new Traversal<HTML*Element> API. This makes the
code shorter / simpler.

R=esprehn, adamk
BUG=346733

Review URL: https://codereview.chromium.org/189523003

git-svn-id: svn://svn.chromium.org/blink/trunk@168792 bbb929c8-8fbe-4397-9dbb-9b2b20218538