Add support for single sample metrics.

components/metrics/single_value_histograms_factory_impl.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_HISTOGRAM_FACTORY_IMPL_H_
+#define COMPONENTS_METRICS_SINGLE_VALUE_HISTOGRAM_FACTORY_IMPL_H_
+
+#include "base/metrics/single_value_histograms.h"
+#include "components/metrics/public/interfaces/single_value_histograms.mojom.h"
+
+namespace metrics {
+
+class BASE_EXPORT SingleValueHistogramsFactoryImpl
+    : public base::SingleValueHistogramsFactory {
+ public:
+  SingleValueHistogramsFactoryImpl(
+      scoped_refptr<base::SingleThreadTaskRunner> task_runner,
+      mojom::SingleValueHistogramsProviderPtr metric_provider);
+  ~SingleValueHistogramsFactoryImpl() override;
+
+  // base::SingleValueHistogramsFactory implementation.
+  std::unique_ptr<base::SingleValueCountsHistogram>
+  CreateSingleValueCountsHistogram(const std::string& name,
+                                   base::HistogramBase::Sample min,
+                                   base::HistogramBase::Sample max,
+                                   uint32_t bucket_count) override;
+
+ private:
+  mojom::SingleValueHistogramsProviderPtr provider_;
+
+  DISALLOW_COPY_AND_ASSIGN(SingleValueHistogramsFactoryImpl);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_SINGLE_VALUE_HISTOGRAM_FACTORY_IMPL_H_