[GStreamer] Audio/Video sink management is incoherent
https://bugs.webkit.org/show_bug.cgi?id=125304

Reviewed by Gustavo Noronha Silva.

Allow subclasses of MediaPlayerPrivateGStreamerBase to create
custom audio/video sinks in a coherent manner using
create{Audio,Video}Sink methods. Convenience getters are also
available. Also removed some un-needed member variables in the
playbin-based player.

No new tests, existing media tests cover this change.

* platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp:
(WebCore::MediaPlayerPrivateGStreamer::MediaPlayerPrivateGStreamer):
(WebCore::MediaPlayerPrivateGStreamer::updateStates):
(WebCore::MediaPlayerPrivateGStreamer::createAudioSink):
(WebCore::MediaPlayerPrivateGStreamer::audioSink):
(WebCore::MediaPlayerPrivateGStreamer::createGSTPlayBin):
* platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.h:
* platform/graphics/gstreamer/MediaPlayerPrivateGStreamerBase.cpp:
* platform/graphics/gstreamer/MediaPlayerPrivateGStreamerBase.h:
(WebCore::MediaPlayerPrivateGStreamerBase::createAudioSink):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@160214 268f45cc-cd09-0410-ab3c-d52691b4dbfc