Fixing WebRtcBrowserTest.CallAndModifyStream, for new addStream behavior.

Soon, the peer connection will stop keeping track of modifications to an
added stream. So to add/remove tracks from a peer connection, you *must*
explicitly call addStream/removeStream.

Review URL: https://codereview.chromium.org/1435713002

Cr-Commit-Position: refs/heads/master@{#359433}
1 file changed