Use std::string references instead of copying contents.

talk/media/webrtc/webrtcvideoengine2.cc

 /*
  * libjingle
  * Copyright 2014 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *
  *  1. Redistributions of source code must retain the above copyright notice,
  *     this list of conditions and the following disclaimer.
  *  2. Redistributions in binary form must reproduce the above copyright notice,
@@ skipping 136 matching lines @@
   // A list of encoders that were created without being wrapped in a
   // SimulcastEncoderAdapter.
   std::vector<webrtc::VideoEncoder*> non_simulcast_encoders_;
 };
 
 bool CodecIsInternallySupported(const std::string& codec_name) {
   if (CodecNamesEq(codec_name, kVp8CodecName)) {
     return true;
   }
   if (CodecNamesEq(codec_name, kVp9CodecName)) {
-    const std::string group_name =
+    const std::string& group_name =
         webrtc::field_trial::FindFullName("WebRTC-SupportVP9");

[pthatcher1, 2015/07/16 09:45:48]

I'm not sure this one is safe, either.  webrtc::field_trial::FindFullName can
return "std::string()", which I don't think is safe to take a reference to.

     return group_name == "Enabled" || group_name == "EnabledByFlag";
   }
   if (CodecNamesEq(codec_name, kH264CodecName)) {
     return webrtc::H264Encoder::IsSupported() &&
         webrtc::H264Decoder::IsSupported();
   }
   return false;
 }
 
 void AddDefaultFeedbackParams(VideoCodec* codec) {
@@ skipping 2434 matching lines @@
       video_codecs[i].rtx_payload_type = rtx_mapping[video_codecs[i].codec.id];
     }
   }
 
   return video_codecs;
 }
 
 }  // namespace cricket
 
 #endif  // HAVE_WEBRTC_VIDEO