SetRemoteDescriptionObserverInterface added.

The new observer replaced SetSessionDescriptionObserver for
SetRemoteDescription. Unlike SetSessionDescriptionObserver,
SetRemoteDescriptionObserverInterface is invoked synchronously so
that the you can rely on the state of the PeerConnection to represent
the result of the SetRemoteDescription call in the callback.

The new observer succeeds or fails with an RTCError.

This deprecates the need for PeerConnectionObserver::OnAdd/RemoveTrack
and SetSessionDescriptionObserver, with the benefit that all media
object changes can be processed in a single callback by the application
in a synchronous callback. This will help Chromium keep objects in-sync
across layers and threads in a non-racy and straight-forward way, see
design doc (Proposal 2):
https://docs.google.com/a/google.com/document/d/1-cDDC82mgU5zrHacfFz720p3xwRtuBkOPSRchh07Ho0/edit?usp=sharing

An adapter for SetSessionDescriptionObserver is added to allow calling
the old SetRemoteDescription signature and get the old behavior
(OnSuccess/OnFailure callback in a Post) until third parties switch.

Bug: webrtc:8473
Change-Id: I3d4eb60da6dd34615f2c9f384aeaf4634e648c99
Reviewed-on: https://webrtc-review.googlesource.com/17523
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Peter Thatcher <pthatcher@webrtc.org>
Reviewed-by: Guido Urdaneta <guidou@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20841}
diff --git a/api/peerconnectioninterface.h b/api/peerconnectioninterface.h
index a682459..0cff5df 100644
--- a/api/peerconnectioninterface.h
+++ b/api/peerconnectioninterface.h
@@ -82,6 +82,7 @@
 #include "api/rtceventlogoutput.h"
 #include "api/rtpreceiverinterface.h"
 #include "api/rtpsenderinterface.h"
+#include "api/setremotedescriptionobserverinterface.h"
 #include "api/stats/rtcstatscollectorcallback.h"
 #include "api/statstypes.h"
 #include "api/turncustomizer.h"
@@ -726,8 +727,18 @@
   // Sets the remote session description.
   // The PeerConnection takes the ownership of |desc| even if it fails.
   // The |observer| callback will be called when done.
+  // TODO(hbos): Remove when Chrome implements the new signature.
   virtual void SetRemoteDescription(SetSessionDescriptionObserver* observer,
-                                    SessionDescriptionInterface* desc) = 0;
+                                    SessionDescriptionInterface* desc) {
+    SetRemoteDescription(
+        std::unique_ptr<SessionDescriptionInterface>(desc),
+        rtc::scoped_refptr<SetRemoteDescriptionObserverInterface>(
+            new SetRemoteDescriptionObserverAdapter(observer)));
+  }
+  // TODO(hbos): Make pure virtual when Chrome has updated its signature.
+  virtual void SetRemoteDescription(
+      std::unique_ptr<SessionDescriptionInterface> desc,
+      rtc::scoped_refptr<SetRemoteDescriptionObserverInterface> observer) {}
   // Deprecated; Replaced by SetConfiguration.
   // TODO(deadbeef): Remove once Chrome is moved over to SetConfiguration.
   virtual bool UpdateIce(const IceServers& configuration,