Don't use result of "field_trial::FindFullName" as string reference.

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 121db147cca3dd40d218f37691f75967969509e8..91997954d61b521458bdcc5d3d539b0ce7e779fe 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";
   }