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Fixed the crash when adding a chromium sink to remote audio track or adding a track to peerconnection more than once.

NOTRY=true

BUG=396447
TEST=open peerConnection demo, http://googlechrome.github.io/webrtc/samples/web/content/peerconnection/
2. start a call
3. Open JS console and input below codes:
# adding a chromium sink to remote audio track:
var v = new webkitMediaStream();
v.addTrack(pc2.getRemoteStreams()[0].getAudioTracks()[0]);
var a = document.createElement('audio');
a.src = URL.createObjectURL(v);

# add a track to peer connection more than once:
var v = new webkitMediaStream();
v.addTrack(pc2.getRemoteStreams()[0].getAudioTracks()[0]);
pc1.addStream(v);

Review URL: https://codereview.chromium.org/411903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285528 0039d316-1c4b-4281-b951-d872f2087c98