Only cast to HTMLInputElement or HTMLTextAreaElement if it's safe.

Removes some FIXMEs.

BUG=none

Review URL: https://codereview.chromium.org/27417003

git-svn-id: svn://svn.chromium.org/blink/trunk@159837 bbb929c8-8fbe-4397-9dbb-9b2b20218538