Use new is*Element() helper functions even more in HTML code

Use new is*Element() helpers even more in HTML code. This makes the code
clearer and simpler. Also use the new Traversal<*Element> API when suitable.

R=eseidel, adamk
BUG=346095

Review URL: https://codereview.chromium.org/198553002

git-svn-id: svn://svn.chromium.org/blink/trunk@169166 bbb929c8-8fbe-4397-9dbb-9b2b20218538