Remove WebMediaPlayer::supportsFullscreen()

All the implementations on the Chromium side simply return true, so
simplify things as much as possible. Leave only traces of this in
HTMLVideoElement to preserve the old behavior, to be removed together
with the rest of that prefixed fullscreen API.

BUG=334985

Review URL: https://codereview.chromium.org/139943006

git-svn-id: svn://svn.chromium.org/blink/trunk@165243 bbb929c8-8fbe-4397-9dbb-9b2b20218538