MediaStream API: Move responsibility for stopping sources to embedder.
Blink should not anymore stopp sources when MediaStream::stop() is called;
this should (and already is) done by Chromium.

BUG=294145

Review URL: https://codereview.chromium.org/26709007

git-svn-id: svn://svn.chromium.org/blink/trunk@159331 bbb929c8-8fbe-4397-9dbb-9b2b20218538