Rename WebRTCRtpContributingSource to WebRTCRtpSource.

This is to account for the fact that this class is used both for CSRCs
(contributing sources) and SSRCs (synchronization sources), not just
CSRCs.

Also moving WebRTCRtpContributingSourceType to WebRTCRtpSource::Type
and adding the "k" prefix to its enum labels.

TBR=foolip@chromium.org

Bug: 893158
Change-Id: I53fb7e6ee0ad46d08e63e6344eb2cf5bac1f900f
Reviewed-on: https://chromium-review.googlesource.com/c/1400835
Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621665}
18 files changed