Change error handlers for Set*Description to use RTCError

Needed in order to return error codes to Chromium.

Bug: chromium:819629, chromium:589455
Change-Id: Iab22250db62a348eee21c6d8bfc44020a7380586
Reviewed-on: https://webrtc-review.googlesource.com/60522
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22367}
diff --git a/pc/peerconnection.h b/pc/peerconnection.h
index f5b53ae..9051910 100644
--- a/pc/peerconnection.h
+++ b/pc/peerconnection.h
@@ -382,10 +382,10 @@
   void PostSetSessionDescriptionSuccess(
       SetSessionDescriptionObserver* observer);
   void PostSetSessionDescriptionFailure(SetSessionDescriptionObserver* observer,
-                                        const std::string& error);
+                                        RTCError&& error);
   void PostCreateSessionDescriptionFailure(
       CreateSessionDescriptionObserver* observer,
-      const std::string& error);
+      RTCError error);
 
   // Synchronous implementations of SetLocalDescription/SetRemoteDescription
   // that return an RTCError instead of invoking a callback.