Add support for single sample metrics.

components/metrics/single_value_counts_histogram.cc

+// 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.
+
+#include "components/metrics/single_value_counts_histogram.h"
+
+#include "base/metrics/histogram.h"
+
+namespace metrics {
+
+SingleValueCountsHistogram::SingleValueCountsHistogram(
+    const std::string& name,
+    base::HistogramBase::Sample min,
+    base::HistogramBase::Sample max,
+    uint32_t bucket_count)
+    : histogram_(base::Histogram::FactoryGet(
+          name,
+          min,
+          max,
+          bucket_count,
+          base::HistogramBase::kUmaTargetedHistogramFlag)) {
+  DCHECK(histogram_);
+  LOG(ERROR) << __func__;
+}
+
+SingleValueCountsHistogram::~SingleValueCountsHistogram() {
+  LOG(ERROR) << __func__;
+  if (sample_ < 0)
+    return;
+  histogram_->Add(sample_);
+}
+
+void SingleValueCountsHistogram::SetSample(base::HistogramBase::Sample sample) {
+  sample_ = sample;
+}
+
+}  // namespace metrics