Extended proxy abstraction, to call certain methods to the worker thread.

Extracted from cl https://codereview.webrtc.org/1766653002/, where
AddOrUpdateSink results in a deadlock.

BUG=webrtc:5426

Review URL: https://codereview.webrtc.org/1861633002

Cr-Original-Commit-Position: refs/heads/master@{#12281}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5b68ab50bbd6ada3725aa1aeac3e87242986b6be
14 files changed