Ignore MediaController in the video fullscreen button logic

There were three problems:

 1. When fullScreenEnabled() was false, HTMLMediaElement's
    enterFullscreen() was used as a fallback, but that function checks
    the setting and does nothing. This has been the case since cleanup
    from April 2013, r149489: https://codereview.chromium.org/14670004

 2. MediaController's hasVideo() returned true if any of the slave
    elements had video, which could show the button for a video element
    with no video.

 3. The fullscreen button didn't do anything for slaved media elements.

The added tests verify that (1) no fullscreen button is shown when the
fullscreen feature is disabled or (2) when there is no video track, and
(3) that clicking the fullscreen button actually enters fullscreen.

BUG=351661

Review URL: https://codereview.chromium.org/196793021

git-svn-id: svn://svn.chromium.org/blink/trunk@169429 bbb929c8-8fbe-4397-9dbb-9b2b20218538