Rename BEGIN_PROXY_MAP --> BEGIN_SIGNALLING_PROXY_MAP.

And BEGIN_WORKER_PROXY_MAP --> BEGIN_PROXY_MAP.

This rename was suggested by Tommi, with the idea that a proxy
invoking methods on the worker thread should be the common case.
It's a followup to https://codereview.webrtc.org/1861633002/

This cl also adds unittests for proxy calls to the
worker thread.

BUG=webrtc:5426

Review URL: https://codereview.webrtc.org/1871833002

Cr-Original-Commit-Position: refs/heads/master@{#12374}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 72c8d2b708d442d16700229fe22918d187534430
11 files changed