Use isHTML*Element() helpers more in the accessibility code

Use isHTML*Element() helpers more in the accessibility code for clarity.

This CL also updates the isHTML*Element() helpers to accept a null pointer
argument instead of asserting. This seems a bit more convenient.

R=esprehn, adamk
BUG=346095

Review URL: https://codereview.chromium.org/189963007

git-svn-id: svn://svn.chromium.org/blink/trunk@168772 bbb929c8-8fbe-4397-9dbb-9b2b20218538