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Extracts RTCRtpSendParameters from RTCRtpParameters.
Extracts RTCRtpEncodingParameters from RTCRtpCodingParameters.
Adds required to many fields and update tests accordindly.

Bug: 803494
Change-Id: I416c0884abf15ab8d55ce5ae481f15ac8012c73d
Reviewed-on: https://chromium-review.googlesource.com/1114844Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Florent Castelli <orphis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570820}