Report expected task queueing time via UMA

components/scheduler/base/task_queue_manager.h

Index: components/scheduler/base/task_queue_manager.h
diff --git a/components/scheduler/base/task_queue_manager.h b/components/scheduler/base/task_queue_manager.h
index 1e9eef4d2819447ae41d96387b56857ee5519f91..a871481ae0020a8cbf355cd758d0c23f4db44714 100644
--- a/components/scheduler/base/task_queue_manager.h
+++ b/components/scheduler/base/task_queue_manager.h
@@ -38,6 +38,7 @@ class LazyNow;
 class RealTimeDomain;
 class TimeDomain;
 class TaskQueueManagerDelegate;
+class TaskTimeTracker;
 
 // The task queue manager provides N task queues and a selector interface for
 // choosing which task queue to service next. Each task queue consists of two
@@ -82,6 +83,10 @@ class SCHEDULER_EXPORT TaskQueueManager
   // tasks posted to the main loop. The batch size is 1 by default.
   void SetWorkBatchSize(int work_batch_size);
 
+  void SetTaskTimeTracker(TaskTimeTracker* task_time_tracker) {

[alex clarke (OOO till 29th), 2016/07/06 09:51:39]

Please add a brief comment explaining what happens if a TaskTimeTracker is
added.  Please also document that it must outlive the TQM (or be removed by
calling SetTaskTimeTracker(nullptr).

[tdresser, 2016/07/06 13:25:34]

Done.

+    task_time_tracker_ = task_time_tracker;
+  }
+
   // These functions can only be called on the same thread that the task queue
   // manager executes its tasks on.
   void AddTaskObserver(base::MessageLoop::TaskObserver* task_observer);
@@ -167,7 +172,8 @@ class SCHEDULER_EXPORT TaskQueueManager
   // run and |should_trigger_wakeup|. Call with an empty |previous_task| if no
   // task was just run.
   void UpdateWorkQueues(bool should_trigger_wakeup,
-                        const internal::TaskQueueImpl::Task* previous_task);
+                        const internal::TaskQueueImpl::Task* previous_task,
+                        LazyNow lazy_now);
 
   // Chooses the next work queue to service. Returns true if |out_queue|
   // indicates the queue from which the next task should be run, false to
@@ -226,7 +232,7 @@ class SCHEDULER_EXPORT TaskQueueManager
 
   bool task_was_run_on_quiescence_monitored_queue_;
 
-  // To reduce locking overhead we track pending calls to DoWork seperatly for
+  // To reduce locking overhead we track pending calls to DoWork seperately for

[Sami, 2016/07/06 10:08:29]

typo: separately :)

[tdresser, 2016/07/06 13:25:34]

Done.

   // the main thread and other threads.
   std::set<base::TimeTicks> main_thread_pending_wakeups_;
 
@@ -239,6 +245,8 @@ class SCHEDULER_EXPORT TaskQueueManager
 
   base::ObserverList<base::MessageLoop::TaskObserver> task_observers_;
 
+  TaskTimeTracker* task_time_tracker_;  // NOT OWNED
+
   const char* tracing_category_;
   const char* disabled_by_default_tracing_category_;
   const char* disabled_by_default_verbose_tracing_category_;