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/peerconnectionfactoryproxy.h b/webrtc/api/peerconnectionfactoryproxy.h
index a25e93b..ef47cdb 100644
--- a/webrtc/api/peerconnectionfactoryproxy.h
+++ b/webrtc/api/peerconnectionfactoryproxy.h
@@ -20,7 +20,7 @@
 
 namespace webrtc {
 
-BEGIN_PROXY_MAP(PeerConnectionFactory)
+BEGIN_SIGNALING_PROXY_MAP(PeerConnectionFactory)
   PROXY_METHOD1(void, SetOptions, const Options&)
   // Can't use PROXY_METHOD5 because scoped_ptr must be moved.
   // TODO(tommi,hbos): Use of templates to support scoped_ptr?
@@ -93,7 +93,7 @@
     return c_->CreatePeerConnection(a1, std::move(ptr_a3), std::move(ptr_a4),
                                     a5);
   }
-  END_PROXY()
+  END_SIGNALING_PROXY()
 
 }  // namespace webrtc