Modify the parameter type of PeerConnectionObserver callback OnAddTrack.

webrtc/api/peerconnectioninterface_unittest.cc

Index: webrtc/api/peerconnectioninterface_unittest.cc
diff --git a/webrtc/api/peerconnectioninterface_unittest.cc b/webrtc/api/peerconnectioninterface_unittest.cc
index 0726c75d4d1e0229da2fd63d312064747a769c7c..d673b4174a93a7a46aa2771477073b4c0b22ffe1 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -562,9 +562,10 @@ class MockPeerConnectionObserver : public PeerConnectionObserver {
     callback_triggered_ = true;
   }
 
-  void OnAddTrack(rtc::scoped_refptr<webrtc::RtpReceiverInterface> receiver,
-                  std::vector<rtc::scoped_refptr<webrtc::MediaStreamInterface>>
-                      streams) override {
+  void OnAddTrack(
+      rtc::scoped_refptr<webrtc::RtpReceiverInterface> receiver,
+      const std::vector<rtc::scoped_refptr<webrtc::MediaStreamInterface>>&
+          streams) override {
     EXPECT_TRUE(receiver != nullptr);
     num_added_tracks_++;
     last_added_track_label_ = receiver->id();