Add implementation in metrics.h that uses atomic pointer.

webrtc/test/histogram.h

 /*
  *  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.
  */
 
 #ifndef WEBRTC_TEST_HISTOGRAM_H_
 #define WEBRTC_TEST_HISTOGRAM_H_
 
 #include <string>
 
 namespace webrtc {
 namespace test {
 
 // Returns the last added sample to a histogram (or -1 if the histogram is not
 // found).
 int LastHistogramSample(const std::string& name);
 
 // Returns the number of added samples to a histogram.
 int NumHistogramSamples(const std::string& name);
 
-// Removes all histograms.
+// Removes all histogram samples.
 void ClearHistograms();
 
 }  // namespace test
 }  // namespace webrtc
 
 #endif  // WEBRTC_TEST_HISTOGRAM_H_