Replace Element::isMediaElement() with isHTMLMediaElement

This make HTMLMediaElement type casting more in line with the generated
code for most other HTML elements. Not incidentally, this simplifies the
code in several places.

BUG=none

Review URL: https://codereview.chromium.org/196723014

git-svn-id: svn://svn.chromium.org/blink/trunk@169293 bbb929c8-8fbe-4397-9dbb-9b2b20218538