Implement PeerConnection::AddTrack/RemoveTrack for Unified Plan

Bug: webrtc:7600
Change-Id: I2a48426a29ac67b6bdbd7817fe07273cdd5fd980
Reviewed-on: https://webrtc-review.googlesource.com/31647
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Peter Thatcher <pthatcher@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21305}
diff --git a/pc/test/mockpeerconnectionobservers.h b/pc/test/mockpeerconnectionobservers.h
index b45c67d..5eb29f4 100644
--- a/pc/test/mockpeerconnectionobservers.h
+++ b/pc/test/mockpeerconnectionobservers.h
@@ -158,6 +158,9 @@
     return candidates;
   }
 
+  bool negotiation_needed() const { return renegotiation_needed_; }
+  void clear_negotiation_needed() { renegotiation_needed_ = false; }
+
   rtc::scoped_refptr<PeerConnectionInterface> pc_;
   PeerConnectionInterface::SignalingState state_;
   std::vector<std::unique_ptr<IceCandidateInterface>> candidates_;