Add UMA for how long service workers run for.

content/browser/service_worker/service_worker_lifetime_tracker.cc

+// 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 "content/browser/service_worker/service_worker_lifetime_tracker.h"
+
+#include "base/bind.h"
+#include "base/time/default_tick_clock.h"
+#include "content/browser/service_worker/service_worker_metrics.h"
+
+namespace content {
+
+constexpr base::TimeDelta kTimerDuration = base::TimeDelta::FromMinutes(5);

[nhiroki, 2017/02/23 07:54:23]

Can you add a comment about why we chose 5 mins?

[falken, 2017/02/23 08:18:18]

Good idea. Done.

+
+ServiceWorkerLifetimeTracker::ServiceWorkerLifetimeTracker()
+    : ServiceWorkerLifetimeTracker(
+          base::WrapUnique(new base::DefaultTickClock)) {}

[nhiroki, 2017/02/23 07:54:23]

base::MakeUnique<base::DefaultTickClock>()?

[falken, 2017/02/23 08:18:18]

Done.

+
+ServiceWorkerLifetimeTracker::ServiceWorkerLifetimeTracker(
+    std::unique_ptr<base::TickClock> tick_clock)
+    : tick_clock_(std::move(tick_clock)),
+      timer_(tick_clock_.get()),
+      weak_factory_(this) {}
+
+ServiceWorkerLifetimeTracker::~ServiceWorkerLifetimeTracker() {}

[nhiroki, 2017/02/23 07:54:23]

= default;

[falken, 2017/02/23 08:18:18]

Done.

+
+void ServiceWorkerLifetimeTracker::StartTiming(int64_t version_id) {
+  DCHECK(running_workers_.find(version_id) == running_workers_.end());

[nhiroki, 2017/02/23 07:54:23]

DCHECK(!base::ContainsKey(running_workers_, version_id)); could be simpler.

[falken, 2017/02/23 08:18:18]

Done.

+
+  running_workers_[version_id] = tick_clock_->NowTicks();
+  if (!timer_.IsRunning()) {
+    timer_.Start(FROM_HERE, kTimerDuration,
+                 base::Bind(&ServiceWorkerLifetimeTracker::RecordHistograms,
+                            weak_factory_.GetWeakPtr()));

[nhiroki, 2017/02/23 07:54:23]

I wonder if we don't have to make |this| weak because timer_ should be stopped
on the dtor of |this|.

[falken, 2017/02/23 08:18:18]

Ah I think you're right. Done.

+  }
+}
+
+void ServiceWorkerLifetimeTracker::StopTiming(int64_t version_id) {
+  auto it = running_workers_.find(version_id);
+  // The entry may have been removed by AbortTiming.
+  if (it == running_workers_.end())
+    return;
+  ServiceWorkerMetrics::RecordRuntime(tick_clock_->NowTicks() - it->second);
+  running_workers_.erase(it);
+}
+
+void ServiceWorkerLifetimeTracker::AbortTiming(int64_t version_id) {
+  auto it = running_workers_.find(version_id);
+  // The entry may have been removed by AbortTiming.
+  if (it == running_workers_.end())
+    return;
+  running_workers_.erase(it);
+}
+
+void ServiceWorkerLifetimeTracker::RecordHistograms() {
+  base::TimeTicks now = tick_clock_->NowTicks();
+
+  for (auto& item : running_workers_) {
+    base::TimeDelta runtime = now - item.second;
+    if (runtime > kTimerDuration)
+      ServiceWorkerMetrics::RecordStillRunning(runtime);
+  }
+
+  if (running_workers_.empty())
+    timer_.Stop();
+}
+
+}  // namespace content