[GTK] Should use GStreamer codec installation infrastructure
https://bugs.webkit.org/show_bug.cgi?id=34085

Reviewed by Martin Robinson.

Initial support for the GStreamer codec installer. The player will
handle missing-plugins messages and use the pbutils codec
installer facilities to install the missing GStreamer
plugins. Once the plugins are installed reset the pipeline state.

* platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp:
(WebCore::mediaPlayerPrivatePluginInstallerResultFunction): This
method is used to notify the player that the missing plugins were installed.
(WebCore):
(WebCore::MediaPlayerPrivateGStreamer::MediaPlayerPrivateGStreamer):
(WebCore::MediaPlayerPrivateGStreamer::handleMessage): Ignore
errors while installing plugins and handle the missing-plugin message.
(WebCore::MediaPlayerPrivateGStreamer::handlePluginInstallerResult):
This method is invoked after the installer finished its task.
* platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.h:
(MediaPlayerPrivateGStreamer):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147279 268f45cc-cd09-0410-ab3c-d52691b4dbfc