Add UMA for tracking which BWE versions are in use.

webrtc/modules/congestion_controller/delay_based_bwe.h

 /*
  *  Copyright (c) 2016 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 115 matching lines @@
   Clock* const clock_;
   RemoteBitrateObserver* const observer_;
   std::unique_ptr<InterArrival> inter_arrival_;
   std::unique_ptr<OveruseEstimator> estimator_;
   OveruseDetector detector_;
   RateStatistics incoming_bitrate_;
   std::list<Probe> probes_;
   size_t total_probes_received_;
   int64_t first_packet_time_ms_;
   int64_t last_update_ms_;
+  bool uma_recorded_;
 
   rtc::CriticalSection crit_;
   Ssrcs ssrcs_ GUARDED_BY(&crit_);
   AimdRateControl remote_rate_ GUARDED_BY(&crit_);
 
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(DelayBasedBwe);
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_CONGESTION_CONTROLLER_DELAY_BASED_BWE_H_