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

Use new is*Element() helpers more in editing 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/196683003

git-svn-id: svn://svn.chromium.org/blink/trunk@169281 bbb929c8-8fbe-4397-9dbb-9b2b20218538