Implement RTCRtpSender/Receiver.getCapabilities()

Intent: https://groups.google.com/a/chromium.org/d/msg/blink-dev/_ktwAuFRUAg/mh_ECO41AQAJ

Bug: 857451
Change-Id: I8c1973e0346a9db71c4dc1fbfc82e3396c48c5ad
Reviewed-on: https://chromium-review.googlesource.com/1133386
Commit-Queue: Florent Castelli <orphis@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576562}
23 files changed