Replace HTMLMediaElement::isVideo() with isHTMLVideoElement

One type testing mechanism to rule them all.

BUG=none

Review URL: https://codereview.chromium.org/200713004

git-svn-id: svn://svn.chromium.org/blink/trunk@169407 bbb929c8-8fbe-4397-9dbb-9b2b20218538