[GStreamer] the GstPlayFlags enum diverged from upstream
https://bugs.webkit.org/show_bug.cgi?id=128957

Reviewed by Philippe Normand.

Removed the GstPlayFlags from the GStreamer implementation and
replaced by the use of the GFlags.

* platform/graphics/gstreamer/GStreamerUtilities.h:
* platform/graphics/gstreamer/GStreamerUtilities.cpp:
(WebCore::getGstPlaysFlag): Created to get the flags by using the
GFlags infrastructure.
* platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp:
(WebCore::MediaPlayerPrivateGStreamer::setDownloadBuffering):
Replaced GST_PLAY_FLAG_DOWNLOAD with getGstPlaysFlag.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@164362 268f45cc-cd09-0410-ab3c-d52691b4dbfc