Stop video capturers in multi-stream test.
Stop video capturers in multi-stream test.

Expected to reduce runtime and flakiness in
CallTest.SendsAndReceivesMultipleStreams on linux_memcheck which is
presumed to be due to contention between the threads.

BUG=
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/5249004

git-svn-id: 917f5d3ca488f358c4d40eaec14422cf392ccec9@5238 4adac7df-926f-26a2-2b94-8c16560cd09d

TimothyGu/libilbc,mwgoldsmith/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,mwgoldsmith/ilbc,mwgoldsmith/libilbc,mwgoldsmith/ilbc,ShiftMediaProject/libilbc,mwgoldsmith/ilbc,ShiftMediaProject/libilbc,mwgoldsmith/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,mwgoldsmith/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,mwgoldsmith/ilbc