Use std::string references instead of copying contents.

talk/media/webrtc/webrtcvideoengine2.cc

Index: talk/media/webrtc/webrtcvideoengine2.cc
diff --git a/talk/media/webrtc/webrtcvideoengine2.cc b/talk/media/webrtc/webrtcvideoengine2.cc
index 657475b281dad59f20c67c9219470098b53f5ed0..80982b41d3c241de3d5dabcd96441ec7c30bf532 100644
--- a/talk/media/webrtc/webrtcvideoengine2.cc
+++ b/talk/media/webrtc/webrtcvideoengine2.cc
@@ -154,7 +154,7 @@ bool CodecIsInternallySupported(const std::string& codec_name) {
     return true;
   }
   if (CodecNamesEq(codec_name, kVp9CodecName)) {
-    const std::string group_name =
+    const std::string& group_name =
         webrtc::field_trial::FindFullName("WebRTC-SupportVP9");
     return group_name == "Enabled" || group_name == "EnabledByFlag";
   }