Fix cancelling of device enumeration requests.

TEST= Log using --vmodule=media_stream_manager*=3,media_stream_dispatcher*=3 and make sure the log print out media_stream_manager.cc(316)]CancelRequest(
after
MediaStreamDispatcher::StopEnumerateDevices() while using the webcam on youtube.com/my_webcam?privacy=public

BUG=317534
R=xians@chromium.org

Review URL: https://codereview.chromium.org/72683002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235139 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235139