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

webrtc/api/rtcstats_unittest.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 164 matching lines @@
 }
 
 TEST(RTCStatsDeathTest, InvalidCasting) {
   RTCGrandChildStats stats("grandchild", 0.0);
   EXPECT_DEATH(stats.cast_to<RTCChildStats>(), "");
 }
 
 #endif  // RTC_DCHECK_IS_ON && GTEST_HAS_DEATH_TEST && !defined(WEBRTC_ANDROID)
 
 }  // namespace webrtc