[GTK] Add graphviz debug output for the gstreamer video pipeline.

[GTK] Add graphviz debug output for the gstreamer video pipeline.
https://bugs.webkit.org/show_bug.cgi?id=89660

Patch by Allan Xavier <allan.xavier@undefinedltd.co.uk> on 2012-06-25
Reviewed by Martin Robinson.

Dump the current video pipeline as a graphviz DOT file when the
pipeline state changes or when an error occours.

No new tests as the change is only useful for debugging.

* platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp:
(WebCore::MediaPlayerPrivateGStreamer::handleMessage):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121169 268f45cc-cd09-0410-ab3c-d52691b4dbfc