Drop excessive StartCapture messages in video capture host.

A renderer process is supposed to send StartCapture message for a specific
device only once before a StopCapture message. The DCHECK() is not good
enough to handle compromised renderer process. Drop all StartCapture messages
if that device has been started in video capture host.

BUG=339314
R=jiayl@chromium.org

Review URL: https://codereview.chromium.org/150103011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249577 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249577