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}

webrtc/api/rtpreceiverinterface.h

@@ -38,11 +38,11 @@ class RtpReceiverInterface : public rtc::RefCountInterface {
 };
 
 // Define proxy for RtpReceiverInterface.
-BEGIN_PROXY_MAP(RtpReceiver)
+BEGIN_SIGNALING_PROXY_MAP(RtpReceiver)
 PROXY_CONSTMETHOD0(rtc::scoped_refptr<MediaStreamTrackInterface>, track)
 PROXY_CONSTMETHOD0(std::string, id)
 PROXY_METHOD0(void, Stop)
-END_PROXY()
+END_SIGNALING_PROXY()
 
 }  // namespace webrtc