Revert 235139 "Fix cancelling of device enumeration requests."

> 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

TBR=perkj@chromium.org

Review URL: https://codereview.chromium.org/74753005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235678 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235678