Add support for single sample metrics.

components/metrics/single_value_counts_histogram.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_METRICS_SINGLE_VALUE_COUNTS_HISTOGRAM_H_
+#define COMPONENTS_METRICS_SINGLE_VALUE_COUNTS_HISTOGRAM_H_
+
+#include "base/macros.h"
+#include "base/metrics/histogram_base.h"
+#include "components/metrics/public/interfaces/single_value_histograms.mojom.h"
+
+namespace metrics {
+
+class SingleValueCountsHistogram : public mojom::SingleValueCountsHistogram {
+ public:
+  SingleValueCountsHistogram(const std::string& name,
+                             base::HistogramBase::Sample min,
+                             base::HistogramBase::Sample max,
+                             uint32_t bucket_count);
+  ~SingleValueCountsHistogram() override;
+
+ private:
+  // mojom::SingleValueCountsHistogram implementation.
+  void SetSample(base::HistogramBase::Sample sample) override;
+
+  base::HistogramBase* const histogram_;
+
+  // The last sample provided to SetSample(). We use -1 as a sentinel value to
+  // indicate no sample has been set.
+  base::HistogramBase::Sample sample_ = -1;
+
+  DISALLOW_COPY_AND_ASSIGN(SingleValueCountsHistogram);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_SINGLE_VALUE_COUNTS_HISTOGRAM_H_