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Groups of streams that should be synchronized are signalled through SDP. These should be used rather than synchronizing the first-added video stream to the first-added audio stream implicitly. BUG=webrtc:4667 R=hta@webrtc.org, solenberg@webrtc.org, stefan@webrtc.org TBR=mflodman@webrtc.org Review URL: https://codereview.webrtc.org/1181653002 Cr-Commit-Position: refs/heads/master@{#9586}