Add WebMediaPlayer::exitedFullscreen() and call it from HTMLMediaElement

Previously WebMediaPlayers were only notified when entering fullscreen and
only if they returned true from supportsOverlayFullscreenVideo(). Now there
will always be a call when entering fullscreen, and there's a matching
exitedFullscreen() call. WebMediaPlayerImpl will use these on Android to
switch the video decoder output surface.

This also renames enterFullscreen to enteredFullscreen.

This CL to start using this is https://codereview.chromium.org/1655083002/

BUG=533630

Review URL: https://codereview.chromium.org/1689043002

Cr-Commit-Position: refs/heads/master@{#376029}