Old SetRemoteDescription signature default implementation.

So that third party projects don't still have to implement it when they
switch over to the new signature.

Bug: webrtc:8473
Change-Id: I329814ad6e899def7bad97939e8643380a268f91
Reviewed-on: https://webrtc-review.googlesource.com/26022
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20885}

api/peerconnectioninterface.h

@@ -729,7 +729,7 @@ class PeerConnectionInterface : public rtc::RefCountInterface {
   // 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) {}
   // TODO(hbos): Make pure virtual when Chrome has updated its signature.
   virtual void SetRemoteDescription(
       std::unique_ptr<SessionDescriptionInterface> desc,