Removed RunUntilIdle between Stop and Close in UT, after Valgrind found a NULL addressing.

Reland of: https://codereview.chromium.org/29423003/

Added video capture capabilities retrieval and caching to VideoCaptureManager. 

The local cache of video capture names and capabilities is created in a
codepath starting in EnumerateDevices. The cache is update during 
StartCaptureForClient() and StopCaptureForClient(). 

Also added unittests (http://goo.gl/QQbpXW). 

BUG=309554, 319955
TBR=perkj, ncarter, xians1

Review URL: https://codereview.chromium.org/74703002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235728 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235728