Moving webrtc/stats/ into webrtc/api/ to resolve a circular dependency

webrtc/api/rtcstatsreport.cc

 /*
  *  Copyright 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 75 matching lines @@
   return ConstIterator(rtc::scoped_refptr<const RTCStatsReport>(this),
                        stats_.cbegin());
 }
 
 RTCStatsReport::ConstIterator RTCStatsReport::end() const {
   return ConstIterator(rtc::scoped_refptr<const RTCStatsReport>(this),
                        stats_.cend());
 }
 
 }  // namespace webrtc