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..66fdb14bda5afc7d61da10ee77f1713afd18a1e6
--- /dev/null
+++ b/base/metrics/single_sample_metrics.cc
@@ -0,0 +1,77 @@
+// 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(
+    std::unique_ptr<SingleSampleMetricsFactory> factory) {
+  DCHECK(!g_factory);
+  g_factory = factory.release();
+}
+
+// static
+void SingleSampleMetricsFactory::DeleteFactoryForTesting() {
+  DCHECK(g_factory);
+  delete g_factory;
+  g_factory = nullptr;
+}
+
+std::unique_ptr<SingleSampleMetric>
+DefaultSingleSampleMetricsFactory::CreateCustomCountsMetric(
+    const std::string& histogram_name,
+    HistogramBase::Sample min,
+    HistogramBase::Sample max,
+    uint32_t bucket_count) {
+  return MakeUnique<DefaultSingleSampleMetric>(
+      histogram_name, min, max, bucket_count,
+      HistogramBase::kUmaTargetedHistogramFlag);
+}
+
+DefaultSingleSampleMetric::DefaultSingleSampleMetric(
+    const std::string& histogram_name,
+    HistogramBase::Sample min,
+    HistogramBase::Sample max,
+    uint32_t bucket_count,
+    int32_t flags)
+    : histogram_(Histogram::FactoryGet(histogram_name,
+                                       min,
+                                       max,
+                                       bucket_count,
+                                       flags)) {
+  // Bad construction parameters may lead to |histogram_| being null; DCHECK to
+  // find accidental errors in production. We must still handle the nullptr in
+  // destruction though since this construction may come from another untrusted
+  // process.
+  DCHECK(histogram_);
+}
+
+DefaultSingleSampleMetric::~DefaultSingleSampleMetric() {
+  // |histogram_| may be nullptr if bad construction parameters are given.
+  if (sample_ < 0 || !histogram_)
+    return;
+  histogram_->Add(sample_);
+}
+
+void DefaultSingleSampleMetric::SetSample(HistogramBase::Sample sample) {
+  DCHECK_GE(sample, 0);
+  sample_ = sample;
+}
+
+}  // namespace base