MediaStream API: MediaStreamTrack can now belong to more than one Stream.
MediaStream::addTrack now adds the track directly instead of cloning it
first.

BUG=294145

Review URL: https://codereview.chromium.org/85713002

git-svn-id: svn://svn.chromium.org/blink/trunk@162758 bbb929c8-8fbe-4397-9dbb-9b2b20218538