Report expected task queueing time via UMA

components/scheduler/renderer/queueing_time_estimator.h

+// Copyright 2016 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_SCHEDULER_RENDERER_QUEUEING_TIME_ESTIMATOR_H_
+#define COMPONENTS_SCHEDULER_RENDERER_QUEUEING_TIME_ESTIMATOR_H_
+
+#include "base/macros.h"
+#include "base/message_loop/message_loop.h"
+#include "base/time/time.h"
+#include "cc/base/rolling_time_delta_history.h"
+#include "components/scheduler/scheduler_export.h"
+
+namespace base {
+class TickClock;
+}
+
+namespace scheduler {
+
+// Records the expected queueing time for a high priority task occurring
+// randomly during each interval of length |window_duration|.
+class SCHEDULER_EXPORT QueueingTimeEstimator
+    : public base::MessageLoop::TaskObserver {
+ public:
+  class Client {
+   public:
+    virtual void OnQueueingTimeForWindowEstimated(
+        base::TimeDelta queueing_time) = 0;
+    virtual ~Client(){};

[Sami, 2016/04/21 10:04:01]

nit: Remove the trailing ';' so git cl format will format this correctly.

[tdresser, 2016/04/21 12:37:28]

Done.

+  };
+
+  QueueingTimeEstimator(Client* client,
+                        base::TickClock* time_source,
+                        base::TimeDelta window_duration);
+  ~QueueingTimeEstimator() override;
+
+  // TaskObserver implementation:
+  void WillProcessTask(const base::PendingTask& pending_task) override;
+  void DidProcessTask(const base::PendingTask& pending_task) override;
+
+ private:
+  Client* client_;  // NOT OWNED.
+  base::TickClock* time_source_;  // NOT OWNED.
+  int outstanding_task_count_;
+
+  base::TimeTicks task_start_time_;
+  base::TimeDelta current_expected_queueing_time_;
+  base::TimeDelta window_duration_;
+  base::TimeTicks window_start_time_;
+
+  DISALLOW_COPY_AND_ASSIGN(QueueingTimeEstimator);
+};
+
+}  // namespace scheduler
+
+#endif  // COMPONENTS_SCHEDULER_RENDERER_QUEUEING_TIME_ESTIMATOR_H_