Allow VideoSender to be constructed on one thread but initialized and used for doing registrations, on another.
Allow VideoSender to be constructed on one thread but initialized and used for doing registrations, on another.

R=andresp@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/42219004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8613}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 658d2015f388664d8327f7da37d157cad5fc981b

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc