Add support for single sample metrics.

base/metrics/single_sample_metrics.cc

Index: base/metrics/single_sample_metrics.cc
diff --git a/base/metrics/single_sample_metrics.cc b/base/metrics/single_sample_metrics.cc
new file mode 100644
index 0000000000000000000000000000000000000000..808ffa06909b7dc4b118f2be92fb8836ac4d43ac
--- /dev/null
+++ b/base/metrics/single_sample_metrics.cc
@@ -0,0 +1,66 @@
+// 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 "base/metrics/single_sample_metrics.h"
+
+#include "base/memory/ptr_util.h"
+#include "base/metrics/histogram.h"
+
+namespace base {
+
+static SingleSampleMetricsFactory* g_factory = nullptr;
+
+// static
+SingleSampleMetricsFactory* SingleSampleMetricsFactory::Get() {
+  if (!g_factory)
+    g_factory = new DefaultSingleSampleMetricsFactory();
+
+  return g_factory;
+}
+
+// static
+void SingleSampleMetricsFactory::SetFactory(
+    SingleSampleMetricsFactory* factory) {
+  DCHECK(!g_factory);
+  g_factory = factory;
+}
+
+// static
+void SingleSampleMetricsFactory::DeleteFactoryForTesting() {
+  DCHECK(g_factory);
+  delete g_factory;
+  g_factory = nullptr;
+}
+
+std::unique_ptr<SingleSampleMetric>
+DefaultSingleSampleMetricsFactory::CreateCustomCountsMetric(
+    const std::string& name,
+    HistogramBase::Sample min,
+    HistogramBase::Sample max,
+    uint32_t bucket_count) {
+  return MakeUnique<DefaultSingleSampleMetric>(
+      name, min, max, bucket_count, HistogramBase::kUmaTargetedHistogramFlag);
+}
+
+DefaultSingleSampleMetric::DefaultSingleSampleMetric(const std::string& name,
+                                                     HistogramBase::Sample min,
+                                                     HistogramBase::Sample max,
+                                                     uint32_t bucket_count,
+                                                     int32_t flags)
+    : histogram_(Histogram::FactoryGet(name, min, max, bucket_count, flags)) {
+  DCHECK(histogram_);
+}
+
+DefaultSingleSampleMetric::~DefaultSingleSampleMetric() {
+  if (sample_ < 0)
+    return;
+  histogram_->Add(sample_);
+}
+
+void DefaultSingleSampleMetric::SetSample(HistogramBase::Sample sample) {
+  DCHECK_GE(sample, 0);
+  sample_ = sample;
+}
+
+}  // namespace base