Reviewed by Maciej Stachowiak.

        AVF: MediaPlayerPrivateAVFoundation never reaches playable state.
        https://bugs.webkit.org/show_bug.cgi?id=57962

        Add support for a new AVPlayerItem API which will notify clients when their
        seek completes.  This requires a new Notification type to be passed to the main
        thread in MediaPlayerPrivateAVFoundation.

        * platform/graphics/avfoundation/MediaPlayerPrivateAVFoundation.cpp:
        (WebCore::MediaPlayerPrivateAVFoundation::seekCompleted): Added.
        (WebCore::MediaPlayerPrivateAVFoundation::scheduleMainThreadNotification): Added two new
            overloaded functions which take a Notification; and a Notification::Type and boolean.
        (WebCore::MediaPlayerPrivateAVFoundation::dispatchNotification): Support new SeekCompleted
            Notification type.
        * platform/graphics/avfoundation/MediaPlayerPrivateAVFoundation.h:
        (WebCore::MediaPlayerPrivateAVFoundation::Notification::Notification): Added one new constructor.
        (WebCore::MediaPlayerPrivateAVFoundation::Notification::finished): Added ivar and accessor.
        * platform/graphics/avfoundation/MediaPlayerPrivateAVFoundationObjC.mm:
        (WebCore::MediaPlayerPrivateAVFoundationObjC::seekToTime): Call new AVPlayerItem API.
        (-[WebCoreAVFMovieObserver seekCompleted:]): Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@83196 268f45cc-cd09-0410-ab3c-d52691b4dbfc