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

"field_trial::FindFullName" can return "std::string()" which should not
be referenced by the caller.

Review URL: https://codereview.webrtc.org/1238943003

Cr-Commit-Position: refs/heads/master@{#9594}
diff --git a/talk/media/webrtc/webrtcvideocapturer.cc b/talk/media/webrtc/webrtcvideocapturer.cc
index 8ef920e..f8c373d 100644
--- a/talk/media/webrtc/webrtcvideocapturer.cc
+++ b/talk/media/webrtc/webrtcvideocapturer.cc
@@ -265,7 +265,7 @@
   // calls, can't take lock.
   DCHECK(module_);
 
-  const std::string& group_name =
+  const std::string group_name =
       webrtc::field_trial::FindFullName("WebRTC-CVO");
 
   if (group_name == "Disabled") {
diff --git a/talk/media/webrtc/webrtcvideoengine2.cc b/talk/media/webrtc/webrtcvideoengine2.cc
index 121db14..9199795 100644
--- a/talk/media/webrtc/webrtcvideoengine2.cc
+++ b/talk/media/webrtc/webrtcvideoengine2.cc
@@ -154,7 +154,7 @@
     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";
   }