Modify the parameter type of PeerConnectionObserver callback OnAddTrack.

webrtc/api/peerconnectioninterface.h

 /*
  *  Copyright 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 585 matching lines @@
       const std::vector<cricket::Candidate>& candidates) {}
 
   // Called when the ICE connection receiving status changes.
   virtual void OnIceConnectionReceivingChange(bool receiving) {}
 
   // Called when a track is added to streams.
   // TODO(zhihuang) Make this a pure virtual method when all its subclasses
   // 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.
   ~PeerConnectionObserver() {}
 };
 
 // PeerConnectionFactoryInterface is the factory interface use for creating
 // PeerConnection, MediaStream and media tracks.
 // PeerConnectionFactoryInterface will create required libjingle threads,
 // socket and network manager factory classes for networking.
@@ skipping 152 matching lines @@
     cricket::WebRtcVideoEncoderFactory* encoder_factory,
     cricket::WebRtcVideoDecoderFactory* decoder_factory) {
   return CreatePeerConnectionFactory(
       worker_and_network_thread, worker_and_network_thread, signaling_thread,
       default_adm, encoder_factory, decoder_factory);
 }
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_PEERCONNECTIONINTERFACE_H_