Cleanup: Pass std::string as const reference from content/

content/renderer/media/webrtc/media_stream_track_metrics_unittest.cc

Index: content/renderer/media/webrtc/media_stream_track_metrics_unittest.cc
diff --git a/content/renderer/media/webrtc/media_stream_track_metrics_unittest.cc b/content/renderer/media/webrtc/media_stream_track_metrics_unittest.cc
index d18c04f9e4ce7c6ec41d4683839fe2933fbc2bae..df840835b0a616fb2ded090c04e7e5cea2e876f1 100644
--- a/content/renderer/media/webrtc/media_stream_track_metrics_unittest.cc
+++ b/content/renderer/media/webrtc/media_stream_track_metrics_unittest.cc
@@ -132,11 +132,11 @@ class MediaStreamTrackMetricsTest : public testing::Test {
   void AddVideoTrack(VideoTrackInterface* track) { AddTrack(track); }
   void RemoveVideoTrack(VideoTrackInterface* track) { RemoveTrack(track); }
 
-  scoped_refptr<MockAudioTrackInterface> MakeAudioTrack(std::string id) {
+  scoped_refptr<MockAudioTrackInterface> MakeAudioTrack(const std::string& id) {
     return new rtc::RefCountedObject<MockAudioTrackInterface>(id);
   }
 
-  scoped_refptr<MockVideoTrackInterface> MakeVideoTrack(std::string id) {
+  scoped_refptr<MockVideoTrackInterface> MakeVideoTrack(const std::string& id) {
     return new rtc::RefCountedObject<MockVideoTrackInterface>(id);
   }