Add UMA for how long service workers run for.

content/browser/service_worker/service_worker_lifetime_tracker.h

+// 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 CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_LIFETIME_TRACKER_H_
+#define CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_LIFETIME_TRACKER_H_
+
+#include <map>
+#include <memory>
+
+#include "base/gtest_prod_util.h"
+#include "base/macros.h"
+#include "base/memory/ptr_util.h"
+#include "base/time/tick_clock.h"
+#include "base/timer/timer.h"
+#include "content/common/content_export.h"
+
+namespace content {
+
+// ServiceWorkerLifetimeTracker tracks how long service workers run, for UMA
+// purposes.
+class CONTENT_EXPORT ServiceWorkerLifetimeTracker {
+ public:
+  ServiceWorkerLifetimeTracker();
+  ServiceWorkerLifetimeTracker(std::unique_ptr<base::TickClock> tick_clock);
+  virtual ~ServiceWorkerLifetimeTracker();
+
+  // Called when the worker started running.
+  void StartTiming(int64_t version_id);

[nhiroki, 2017/02/28 08:50:07]

Can you replace version_id with embedded_worker_id because
EmbeddedWorkerRegistry passes it?

+  // Called when the worker stopped running.
+  void StopTiming(int64_t version_id);
+  // Called when DevTools was attached to the worker. Forgets the outstanding
+  // start timing.
+  void AbortTiming(int64_t version_id);
+
+ private:
+  friend class ServiceWorkerLifetimeTrackerTest;
+
+  void RecordHistograms();
+
+  std::unique_ptr<base::TickClock> tick_clock_;
+  std::map<int64_t /* version_id */, base::TimeTicks /* start_time */>
+      running_workers_;
+  base::RepeatingTimer timer_;
+
+  DISALLOW_COPY_AND_ASSIGN(ServiceWorkerLifetimeTracker);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_LIFETIME_TRACKER_H_