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WebRTCRtpTransceiver added for surfacing transceivers in future CLs.
Implementation added in content and test coverage added.

The content::RTCRtpTransceiver is implemented with a similar design as
content::RTCRtp[Sender/Receiver]:
- It has an "internal" ref counted class, allowing multiple
  WebRTCRtpTransceivers to represent the same webrtc layer transceiver.
- The "state" is in a separate class, allowing surfacing transceiver
  state information separately from implementation, and use of
  TransceiverStateSurfacer.

Stop(), blink-layer RTCRtpTransceiver and Unified Plan usage will be
added in follow-up CLs.

This CL also removes all dependencies on
webrtc_media_stream_adapter_map.[h/cc] unblocking its deletion.

Bug: 777617
Change-Id: Icbc4a6b53ef57df8da6bc10e9e432644368ba5dd
Reviewed-on: https://chromium-review.googlesource.com/1131475Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574945}