Add support for single sample metrics.

base/metrics/single_value_histograms.h

Index: base/metrics/single_value_histograms.h
diff --git a/base/metrics/single_value_histograms.h b/base/metrics/single_value_histograms.h
new file mode 100644
index 0000000000000000000000000000000000000000..c6e98ae358258186e1178550b80b4e6a38c21020
--- /dev/null
+++ b/base/metrics/single_value_histograms.h
@@ -0,0 +1,37 @@
+// 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 BASE_METRICS_SINGLE_VALUE_HISTOGRAMS_H_
+#define BASE_METRICS_SINGLE_VALUE_HISTOGRAMS_H_
+
+#include "base/base_export.h"
+#include "base/macros.h"
+#include "base/metrics/histogram_base.h"
+
+namespace base {
+
+class BASE_EXPORT SingleValueCountsHistogram {
+ public:
+  virtual ~SingleValueCountsHistogram() {}
+
+  virtual void SetSample(base::HistogramBase::Sample sample) = 0;
+};
+
+class BASE_EXPORT SingleValueHistogramsFactory {
+ public:
+  virtual ~SingleValueHistogramsFactory() {}
+
+  static SingleValueHistogramsFactory* Get();
+  static void SetFactory(SingleValueHistogramsFactory* factory);
+
+  virtual std::unique_ptr<SingleValueCountsHistogram>
+  CreateSingleValueCountsHistogram(const std::string& name,
+                                   base::HistogramBase::Sample min,
+                                   base::HistogramBase::Sample max,
+                                   uint32_t bucket_count) = 0;
+};
+
+}  // namespace base
+
+#endif  // BASE_METRICS_SINGLE_VALUE_HISTOGRAMS_H_