Add/remove receive streams with SSRC 0 from media channels

This enables creation and removal of receive streams with SSRC 0.
Several related methods, for example SetOutputVolume, still use 0 as a
special value.

Bug: webrtc:8694
Change-Id: I341e6bd6c981c9838997510d8d712ad2948f6460
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/152780
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Saurav Das <dinosaurav@chromium.org>
Cr-Commit-Position: refs/heads/master@{#29398}
diff --git a/media/base/fake_media_engine.h b/media/base/fake_media_engine.h
index 28af02a..ac303e6 100644
--- a/media/base/fake_media_engine.h
+++ b/media/base/fake_media_engine.h
@@ -117,6 +117,8 @@
     }
     return RemoveStreamBySsrc(&send_streams_, ssrc);
   }
+  virtual void ResetUnsignaledRecvStream() {}
+
   virtual bool AddRecvStream(const StreamParams& sp) {
     if (absl::c_linear_search(receive_streams_, sp)) {
       return false;