Resource leak related to gstreamer and videos

MERGE NOTE: Fixes a leak caused by a circular reference between GStreamer
and WebKit, that triggers whenever a page with HTML5 video has been visited
and it fully loaded.

https://bugs.webkit.org/show_bug.cgi?id=109350

Reviewed by Eric Carlson.

Since active DOM object are only stopped once and can not be restarted, we can
unreference the media player there, instead of waiting for garbage collection.

If the r152778 is rolled back, this also breaks a circular reference between
GStreamer and WebKit which prevented garbage collecting from triggering.

* html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::stop):

Change-Id: Ie13d202d3cac9b97eb158964b33d492e17f817ef
git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153937 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>