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

webrtc/system_wrappers/include/metrics_default.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.
+ */
+
+#ifndef WEBRTC_SYSTEM_WRAPPERS_INCLUDE_METRICS_DEFAULT_H_
+#define WEBRTC_SYSTEM_WRAPPERS_INCLUDE_METRICS_DEFAULT_H_
+
+#include <map>
+#include <memory>
+#include <string>
+
+namespace webrtc {
+namespace metrics {
+
+struct SampleInfo {
+  SampleInfo(const std::string& name, int min, int max, size_t bucket_count);
+  ~SampleInfo();
+
+  const std::string name;
+  const int min;
+  const int max;
+  const size_t bucket_count;
+  std::map<int, int> samples;  // <value, # of events>
+};
+
+// Enables collection of samples.
+// This method should be called before any other call into webrtc.
+void Enable();
+
+// Gets histograms and clears all samples.
+void GetAndReset(
+    std::map<std::string, std::unique_ptr<SampleInfo>>* histograms);
+
+// Functions below are mainly for testing.
+
+// Clears all samples.
+void Reset();
+
+// Returns the number of times the |sample| has been added to the histogram.
+int NumEvents(const std::string& name, int sample);
+
+// Returns the total number of added samples to the histogram.
+int NumSamples(const std::string& name);
+
+// Returns the minimum sample value (or -1 if the histogram has no samples).
+int MinSample(const std::string& name);
+
+}  // namespace metrics
+}  // namespace webrtc
+
+#endif  // WEBRTC_SYSTEM_WRAPPERS_INCLUDE_METRICS_DEFAULT_H_