Add support for single sample metrics.

components/metrics/single_sample_metrics.cc

Index: components/metrics/single_sample_metrics.cc
diff --git a/components/metrics/single_sample_metrics.cc b/components/metrics/single_sample_metrics.cc
new file mode 100644
index 0000000000000000000000000000000000000000..5d6b59e1f10f248c5044f37b935b77897ab410df
--- /dev/null
+++ b/components/metrics/single_sample_metrics.cc
@@ -0,0 +1,84 @@
+// 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_sample_metrics.h"
+
+#include "base/memory/ptr_util.h"
+#include "base/metrics/single_sample_metrics.h"
+#include "base/threading/thread_checker.h"
+#include "components/metrics/single_sample_metrics_factory_impl.h"
+#include "mojo/public/cpp/bindings/strong_binding.h"
+
+namespace metrics {
+
+namespace {
+
+class MojoSingleSampleMetric : public mojom::SingleSampleMetric {
+ public:
+  MojoSingleSampleMetric(const std::string& name,
+                         base::HistogramBase::Sample min,
+                         base::HistogramBase::Sample max,
+                         uint32_t bucket_count,
+                         int32_t flags)
+      : metric_(name, min, max, bucket_count, flags) {}
+  ~MojoSingleSampleMetric() override {}
+
+ private:
+  // mojom::SingleSampleMetric:
+  void SetSample(base::HistogramBase::Sample sample) override {
+    metric_.SetSample(sample);
+  }
+
+  base::DefaultSingleSampleMetric metric_;
+
+  DISALLOW_COPY_AND_ASSIGN(MojoSingleSampleMetric);
+};
+
+class MojoSingleSampleMetricsProvider
+    : public mojom::SingleSampleMetricsProvider {
+ public:
+  MojoSingleSampleMetricsProvider() {}
+  ~MojoSingleSampleMetricsProvider() override {
+    DCHECK(thread_checker_.CalledOnValidThread());
+  }
+
+ private:
+  // mojom::SingleSampleMetricsProvider:
+  void AcquireSingleSampleMetric(
+      const std::string& name,
+      base::HistogramBase::Sample min,
+      base::HistogramBase::Sample max,
+      uint32_t bucket_count,
+      int32_t flags,
+      mojom::SingleSampleMetricRequest request) override {
+    DCHECK(thread_checker_.CalledOnValidThread());
+    mojo::MakeStrongBinding(base::MakeUnique<MojoSingleSampleMetric>(
+                                name, min, max, bucket_count, flags),
+                            std::move(request));
+  }
+
+  // Providers must be created, used on, and destroyed on the same thread.
+  base::ThreadChecker thread_checker_;
+
+  DISALLOW_COPY_AND_ASSIGN(MojoSingleSampleMetricsProvider);
+};
+
+}  // namespace
+
+// static
+void InitializeSingleSampleMetricsFactory(CreateProviderCB create_provider_cb) {
+  base::SingleSampleMetricsFactory::SetFactory(
+      base::MakeUnique<SingleSampleMetricsFactoryImpl>(
+          std::move(create_provider_cb)));
+}
+
+// static
+void CreateSingleSampleMetricsProvider(
+    const service_manager::BindSourceInfo& /* source_info */,

[Alexei Svitkine (slow), 2017/05/04 19:03:21]

Nit: I don't think most of Chrome follows the convention of commenting out
unused params. Certainly we don't in metrics component - so just specify the
param normally.

[DaleCurtis, 2017/05/04 19:08:50]

I can change it, but this is codified in the style guide and we do use it in
media and blink at least:

https://google.github.io/styleguide/cppguide.html#Function_Declarations_and_D...

Happy to change if you prefer otherwise.

[Alexei Svitkine (slow), 2017/05/04 19:14:20]

This has been discussed previously on chromium-dev@ here:

https://groups.google.com/a/chromium.org/d/msg/chromium-dev/TBaVejz_hGQ/5eHIZ...

My understanding is there was a preference for not doing this and that
commenting them out is allowed by style guide simply because some projects
compile themselves with settings where it would otherwise cause a warning. But
Chromium doesn't, so we shouldn't have the extra noise from it in the codebase.

[DaleCurtis, 2017/05/04 21:03:38]

Okay, done! Thanks for the link.

+    mojom::SingleSampleMetricsProviderRequest request) {
+  mojo::MakeStrongBinding(base::MakeUnique<MojoSingleSampleMetricsProvider>(),
+                          std::move(request));
+}
+
+}  // namespace metrics