Add a default implementation in metrics_default.cc of histograms methods in system_wrappers/interfac

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 histogram samples.
-void ClearHistograms();
-
-}  // namespace test
-}  // namespace webrtc
-
-#endif  // WEBRTC_TEST_HISTOGRAM_H_
-