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

Use new is*Element() helpers more in core/ code. This makes the code clearer
and simpler.

R=eseidel, adamk
BUG=346095

Review URL: https://codereview.chromium.org/193623002

git-svn-id: svn://svn.chromium.org/blink/trunk@168969 bbb929c8-8fbe-4397-9dbb-9b2b20218538