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-Commit-Position: refs/heads/master@{#12374}
diff --git a/webrtc/api/rtpsenderinterface.h b/webrtc/api/rtpsenderinterface.h
index 776d01a..2291bb4 100644
--- a/webrtc/api/rtpsenderinterface.h
+++ b/webrtc/api/rtpsenderinterface.h
@@ -60,7 +60,7 @@
 };
 
 // Define proxy for RtpSenderInterface.
-BEGIN_PROXY_MAP(RtpSender)
+BEGIN_SIGNALING_PROXY_MAP(RtpSender)
 PROXY_METHOD1(bool, SetTrack, MediaStreamTrackInterface*)
 PROXY_CONSTMETHOD0(rtc::scoped_refptr<MediaStreamTrackInterface>, track)
 PROXY_METHOD1(void, SetSsrc, uint32_t)
@@ -72,7 +72,7 @@
 PROXY_METHOD0(void, Stop)
 PROXY_CONSTMETHOD0(RtpParameters, GetParameters);
 PROXY_METHOD1(bool, SetParameters, const RtpParameters&)
-END_PROXY()
+END_SIGNALING_PROXY()
 
 }  // namespace webrtc