[GStreamer] Move elements registration to GStreamerCommon

[GStreamer] Move elements registration to GStreamerCommon
https://bugs.webkit.org/show_bug.cgi?id=191189

Reviewed by Xabier Rodriguez-Calvar.

It was a bit odd to have this in the base player class and to have
sub-classes calling a static function of the super-class.

Covered by existing tests.

* platform/graphics/gstreamer/GStreamerCommon.cpp:
(WebCore::initializeGStreamerAndRegisterWebKitElements):
* platform/graphics/gstreamer/GStreamerCommon.h:
* platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp:
(WebCore::MediaPlayerPrivateGStreamer::isAvailable):
(WebCore::MediaPlayerPrivateGStreamer::loadFull):
(WebCore::MediaPlayerPrivateGStreamer::getSupportedTypes):
* platform/graphics/gstreamer/MediaPlayerPrivateGStreamerBase.cpp:
* platform/graphics/gstreamer/MediaPlayerPrivateGStreamerBase.h:
* platform/graphics/gstreamer/mse/MediaPlayerPrivateGStreamerMSE.cpp:
(WebCore::MediaPlayerPrivateGStreamerMSE::registerMediaEngine):
(WebCore::MediaPlayerPrivateGStreamerMSE::load):
(WebCore::MediaPlayerPrivateGStreamerMSE::trackDetected):
(WebCore::MediaPlayerPrivateGStreamerMSE::supportsType):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@237794 268f45cc-cd09-0410-ab3c-d52691b4dbfc