Added metrics to track the time between task switches done via the shelf.

ash/metrics/task_switch_time_tracker.cc

Index: ash/metrics/task_switch_time_tracker.cc
diff --git a/ash/metrics/task_switch_time_tracker.cc b/ash/metrics/task_switch_time_tracker.cc
new file mode 100644
index 0000000000000000000000000000000000000000..036476e5279a182d517f1aa99ef9db431391b524
--- /dev/null
+++ b/ash/metrics/task_switch_time_tracker.cc
@@ -0,0 +1,81 @@
+// Copyright 2015 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 "ash/metrics/task_switch_time_tracker.h"
+
+#include "base/metrics/histogram.h"
+#include "base/time/default_tick_clock.h"
+
+namespace ash {
+
+namespace {
+
+// The number of buckets in the histogram.

[Alexei Svitkine (slow), 2015/05/14 14:51:54]

Please add a comment somewhere that changing any of these values requires
renaming all the histograms using this mechanism.

[bruthig, 2015/05/14 15:35:20]

Done.

+const size_t kBucketCount = 50;
+
+// The underflow (aka minimum) bucket size for the histogram.
+const int kMinBucketSizeInSeconds = 0;
+
+// The overflow (aka maximium) bucket size for the histogram.
+const int kMaxBucketSizeInSeconds = 60 * 60;
+
+}  // namespace
+
+TaskSwitchTimeTracker::TaskSwitchTimeTracker(const std::string& histogram_name)
+    : histogram_name_(histogram_name),
+      tick_clock_(new base::DefaultTickClock()) {
+}
+
+TaskSwitchTimeTracker::TaskSwitchTimeTracker(
+    const std::string& histogram_name,
+    scoped_ptr<base::TickClock> tick_clock)
+    : histogram_name_(histogram_name), tick_clock_(tick_clock.release()) {
+}
+
+TaskSwitchTimeTracker::~TaskSwitchTimeTracker() {
+}
+
+void TaskSwitchTimeTracker::OnTaskSwitch() {
+  if (!HasLastActionTime())
+    SetLastActionTime();
+  else
+    RecordTimeDelta();
+}
+
+bool TaskSwitchTimeTracker::HasLastActionTime() const {
+  return last_action_time_ != base::TimeTicks();
+}
+
+base::TimeTicks TaskSwitchTimeTracker::SetLastActionTime() {
+  base::TimeTicks previous_last_action_time = last_action_time_;
+  last_action_time_ = tick_clock_->NowTicks();
+  return previous_last_action_time;
+}
+
+void TaskSwitchTimeTracker::RecordTimeDelta() {
+  base::TimeTicks previous_last_action_time = SetLastActionTime();
+  base::TimeDelta time_delta = last_action_time_ - previous_last_action_time;
+
+  CHECK_GE(time_delta, base::TimeDelta());
+
+  GetHistogram()->Add(time_delta.InSeconds());
+}
+
+base::HistogramBase* TaskSwitchTimeTracker::GetHistogram() {
+  if (!histogram_) {
+    histogram_ = base::Histogram::FactoryGet(
+        histogram_name_,
+        base::TimeDelta::FromSeconds(kMinBucketSizeInSeconds).InSeconds(),
+        base::TimeDelta::FromSeconds(kMaxBucketSizeInSeconds).InSeconds(),
+        kBucketCount, base::HistogramBase::kUmaTargetedHistogramFlag);
+  }
+
+#if DCHECK_IS_ON()
+  histogram_->CheckName(histogram_name_);
+#endif  // DCHECK_IS_ON()
+
+  return histogram_;
+}
+
+}  // namespace ash