Allow media engine to update HTMLMediaElement state when playing to external device

Allow media engine to update HTMLMediaElement state when playing to external device
https://bugs.webkit.org/show_bug.cgi?id=130480

Reviewed by Jer Noble.

* html/HTMLMediaElement.h: Add mediaPlayerHandlePlaybackCommand.

* platform/graphics/MediaPlayer.cpp:
(WebCore::MediaPlayer::handlePlaybackCommand): New.
* platform/graphics/MediaPlayer.h:
(WebCore::MediaPlayerClient::mediaPlayerHandlePlaybackCommand):

* platform/graphics/avfoundation/MediaPlayerPrivateAVFoundation.cpp:
(WebCore::MediaPlayerPrivateAVFoundation::rateChanged): Send Play and Pause commands when
    playing to an external device.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165928 268f45cc-cd09-0410-ab3c-d52691b4dbfc