Reviewed by Eric Carlson.

        Autoplaying video with poster doesn't reliably show up
        https://bugs.webkit.org/show_bug.cgi?id=34966

        A timing issue with compositing updates when replacing the poster image
        with the video could cause the video to not display.

        Fix by making video layer hook up more similar to WebGL etc, by having the
        video kick off a recalcStyle() via a SyntheticStyleChange. This requires
        vending a PlaformLayer* from the media player, up through the element.

        Test: media/video-replaces-poster.html

        * html/HTMLMediaElement.cpp:
        (WebCore::HTMLMediaElement::mediaPlayerRenderingModeChanged): Use setNeedsStyleRecalc()
        to kick off a compositing update.

        * html/HTMLMediaElement.h:
        (WebCore::HTMLMediaElement::platformLayer): Export the media engine's layer.

        * platform/graphics/MediaPlayer.cpp:
        (WebCore::NullMediaPlayerPrivate::platformLayer): Method to vend the media layer.
        (WebCore::MediaPlayer::platformLayer):

        * platform/graphics/MediaPlayer.h:
        (WebCore::MediaPlayerClient::mediaPlayerRenderingModeChanged): New client callback
        to indicate that the rendering mode changed. The element uses this to kick off a
        recalcStyle.

        * platform/graphics/MediaPlayerPrivate.h:
        (WebCore::MediaPlayerPrivateInterface::platformLayer): Method to vend the media layer.
        * platform/graphics/mac/MediaPlayerPrivateQTKit.h: Ditto
        * platform/graphics/mac/MediaPlayerPrivateQTKit.mm:
        (WebCore::MediaPlayerPrivate::createQTMovieLayer): No longer parent the layer directly
        via setContentsToMedia().
        (WebCore::MediaPlayerPrivate::acceleratedRenderingStateChanged): Ditto.
        (WebCore::MediaPlayerPrivate::setUpVideoRendering): Tell the client that the
        rendering mode changed.
        (WebCore::MediaPlayerPrivate::platformLayer): Method to vend the media layer.
        * rendering/RenderLayerBacking.cpp:
        (WebCore::RenderLayerBacking::updateGraphicsLayerConfiguration): Call setContentsToMedia()
        here.

        * platform/graphics/win/MediaPlayerPrivateQuickTimeWin.h: Add platformLayer().
        * platform/graphics/win/MediaPlayerPrivateQuickTimeWin.cpp:
        (WebCore::MediaPlayerPrivate::platformLayer): Method to vend the media layer.
        (WebCore::MediaPlayerPrivate::setUpVideoRendering): Call mediaPlayerRenderingModeChanged()
        (WebCore::MediaPlayerPrivate::createLayerForMovie): Don't parent the layer ourselves any more.

        * rendering/RenderVideo.h: videoGraphicsLayer() is no longer needed.
        * rendering/RenderVideo.cpp: Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@54826 268f45cc-cd09-0410-ab3c-d52691b4dbfc