Adding some features to proxy.h, and restructuring the macros.

New features are:
- Invoke a destructor on the worker thread.
- Make proxy wrapper for a non-refcounted object.
- Ability to use unique_ptrs (as arguments or return values).

These features are needed by this CL:
https://codereview.webrtc.org/2632613002/

BUG=None

Review-Url: https://codereview.webrtc.org/2628343003
Cr-Commit-Position: refs/heads/master@{#16151}
diff --git a/webrtc/api/videosourceproxy.h b/webrtc/api/videosourceproxy.h
index eaec17b..b678acb 100644
--- a/webrtc/api/videosourceproxy.h
+++ b/webrtc/api/videosourceproxy.h
@@ -21,6 +21,7 @@
 // destroyed on the signaling thread and marshals all method calls to the
 // signaling thread.
 BEGIN_PROXY_MAP(VideoTrackSource)
+  PROXY_SIGNALING_THREAD_DESTRUCTOR()
   PROXY_CONSTMETHOD0(SourceState, state)
   PROXY_CONSTMETHOD0(bool, remote)
   PROXY_CONSTMETHOD0(bool, is_screencast)
@@ -33,7 +34,7 @@
   PROXY_WORKER_METHOD1(void, RemoveSink, rtc::VideoSinkInterface<VideoFrame>*)
   PROXY_METHOD1(void, RegisterObserver, ObserverInterface*)
   PROXY_METHOD1(void, UnregisterObserver, ObserverInterface*)
-END_PROXY()
+END_PROXY_MAP()
 
 }  // namespace webrtc