Add support for single sample metrics.

components/metrics/single_value_histograms_factory_impl.h

Index: components/metrics/single_value_histograms_factory_impl.h
diff --git a/components/metrics/single_value_histograms_factory_impl.h b/components/metrics/single_value_histograms_factory_impl.h
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+++ b/components/metrics/single_value_histograms_factory_impl.h
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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.
+
+#ifndef COMPONENTS_METRICS_SINGLE_VALUE_HISTOGRAM_FACTORY_IMPL_H_
+#define COMPONENTS_METRICS_SINGLE_VALUE_HISTOGRAM_FACTORY_IMPL_H_
+
+#include <string>
+
+#include "base/metrics/single_value_histograms.h"
+#include "base/threading/thread_local.h"
+#include "components/metrics/public/interfaces/single_value_histograms.mojom.h"
+
+namespace base {
+class SingleThreadTaskRunner;
+}
+
+namespace service_manager {
+class Connector;
+}
+
+namespace metrics {
+class SingleValueHistogramsProviderTrampoline;
+
+class SingleValueHistogramsFactoryImpl
+    : public base::SingleValueHistogramsFactory {
+ public:
+  // Constructs a factory capable of vending single value histograms from any
+  // thread. Uses |connector| to create SingleValueHistogramsProvider instances
+  // as necessary per thread. |task_runner| is the thread |connector| must be
+  // used from. Provider instances are stored in thread local storage (TLS).
+  SingleValueHistogramsFactoryImpl(
+      service_manager::Connector* connector,
+      scoped_refptr<base::SingleThreadTaskRunner> task_runner);
+  ~SingleValueHistogramsFactoryImpl() override;
+
+  // base::SingleValueHistogramsFactory implementation.
+  std::unique_ptr<base::SingleValueCountsHistogram>
+  CreateSingleValueCountsHistogram(const std::string& name,
+                                   base::HistogramBase::Sample min,
+                                   base::HistogramBase::Sample max,
+                                   uint32_t bucket_count) override;
+
+ private:
+  // Gets the SingleValueHistogramsProvider for the current thread. If none
+  // exists, then a new instance is created and set in the TLS slot.
+  mojom::SingleValueHistogramsProvider* GetProvider();
+
+  // Service manager connector and the task runner it must be used from.
+  service_manager::Connector* const connector_;
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+
+  // Per thread storage slot for internal trampoline. Necessary since we can't
+  // store mojo-style "pointers" using the TLS helper classes.
+  base::ThreadLocalPointer<SingleValueHistogramsProviderTrampoline> provider_;
+
+  DISALLOW_COPY_AND_ASSIGN(SingleValueHistogramsFactoryImpl);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_SINGLE_VALUE_HISTOGRAM_FACTORY_IMPL_H_