Change RtpSender to have multiple stream_ids

This is part of the work towards implementing Unified Plan.

In Unified Plan, an RtpSender/MediaStreamTrack can be a part of
multiple streams. This changes RtpSender to internally store
stream ids as a vector so that later CL's can update downstream
consumers to support multiple streams. For now, the vector can
only have exactly one element in it, but once the supporting code
has been updated the singular getter/setter will be removed and
any number of streams can be associated with an RtpSender.

Bug: webrtc:8183
Change-Id: I499a77ce79198861d35c91328b40ced2eb913cc0
Reviewed-by: Taylor Brandstetter <>
Commit-Queue: Steve Anton <>
Cr-Original-Commit-Position: refs/heads/master@{#19733}
Cr-Mirrored-Commit: 8ffb9c388f3433eadfb4b532e751f501cb584efd
9 files changed