Add support for single sample metrics.

components/metrics/public/interfaces/single_value_histograms.mojom

Index: components/metrics/public/interfaces/single_value_histograms.mojom
diff --git a/components/metrics/public/interfaces/single_value_histograms.mojom b/components/metrics/public/interfaces/single_value_histograms.mojom
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+// 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.
+
+module metrics.mojom;
+
+interface SingleValueHistogramsProvider {
+  // Returns a SingleValueHistogram for a counts type histogram.
+  //
+  // A single value histogram only reports its sample once at destruction time.
+  // The sample may be changed prior to destruction using the SetSample() method
+  // as many times as desired.
+  //
+  // See base/metrics/histograms.h for parameter definitions. |request| is the
+  // returned histogram.
+  AcquireSingleValueCountsHistogram(string name, int32 min, int32 max,
+                                    uint32 bucket_count,
+                                    SingleValueCountsHistogram& request);
+};
+
+interface SingleValueCountsHistogram {
+  SetSample(int32 sample);
+};