Modify the parameter type of PeerConnectionObserver callback OnAddTrack.

Change the second parameter type to a const reference of vector so that
the vector will not be copied.

BUG=none

Review-Url: https://codereview.webrtc.org/2551603003
Cr-Commit-Position: refs/heads/master@{#15396}
diff --git a/webrtc/api/peerconnectioninterface.h b/webrtc/api/peerconnectioninterface.h
index e14b79c..54fae2b 100644
--- a/webrtc/api/peerconnectioninterface.h
+++ b/webrtc/api/peerconnectioninterface.h
@@ -603,7 +603,7 @@
   // implement it.
   virtual void OnAddTrack(
       rtc::scoped_refptr<RtpReceiverInterface> receiver,
-      std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams) {}
+      const std::vector<rtc::scoped_refptr<MediaStreamInterface>>& streams) {}
 
  protected:
   // Dtor protected as objects shouldn't be deleted via this interface.