Fix bug from r107672.

Fix bug from r107672.
https://bugs.webkit.org/show_bug.cgi?id=78773

Before set MediaStreamTrack disable/enable operation,
checking the status of MediaStream that contains this MediaStreamTrack.
if MediaStream is finished, then skipping operation of
didSetMediaStreamTrackEnabled().

Patch by Lin Wei <wei.a.lin@intel.com> on 2012-02-21
Reviewed by Adam Barth.
* mediastream/MediaStreamTrack.cpp:
(WebCore::MediaStreamTrack::setEnabled):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108383 268f45cc-cd09-0410-ab3c-d52691b4dbfc