Wire up experiment for improved screenshare bwe.

webrtc/video/video_loopback.cc

 /*
  *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 278 matching lines @@
     test.RunWithAnalyzer(params);
   } else {
     test.RunWithRenderers(params);
   }
 }
 }  // namespace webrtc
 
 int main(int argc, char* argv[]) {
   ::testing::InitGoogleTest(&argc, argv);
   google::ParseCommandLineFlags(&argc, &argv, true);
-  webrtc::test::InitFieldTrialsFromString(
+  webrtc::test::ScopedFieldTrials field_trials(
       webrtc::flags::FLAGS_force_fieldtrials);
   webrtc::test::RunTest(webrtc::Loopback);
   return 0;
 }