Use new Traversal<HTML*Element> API in HTML code

Use new Traversal<HTML*Element> API in HTML code. This makes the code simpler
and shorter.

R=esprehn, adamk
BUG=346733

Review URL: https://codereview.chromium.org/189923002

git-svn-id: svn://svn.chromium.org/blink/trunk@168791 bbb929c8-8fbe-4397-9dbb-9b2b20218538