Address comments from cr 43769004.
- Remove unnecessary hop to worker from OnChannelRequestSignaling_s.
- Remove now-not-needed component param.
- Update documentation.

R=juberti@webrtc.org
BUG=4444

Review URL: https://webrtc-codereview.appspot.com/42839004

Cr-Original-Commit-Position: refs/heads/master@{#8852}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 245989b22ac6580d5a42346d7aab7b676dbc457e