scheduler: Implement task observers

content/renderer/scheduler/task_queue_manager.h

Index: content/renderer/scheduler/task_queue_manager.h
diff --git a/content/renderer/scheduler/task_queue_manager.h b/content/renderer/scheduler/task_queue_manager.h
index 0b1bcbba9d619976eca8671d82ea5641e9fe2a1e..9e475ffe13801d67f50d41a6ce5ee3fab9418951 100644
--- a/content/renderer/scheduler/task_queue_manager.h
+++ b/content/renderer/scheduler/task_queue_manager.h
@@ -9,6 +9,7 @@
 #include "base/debug/task_annotator.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
+#include "base/message_loop/message_loop.h"
 #include "base/pending_task.h"
 #include "base/single_thread_task_runner.h"
 #include "base/synchronization/lock.h"
@@ -105,6 +106,11 @@ class CONTENT_EXPORT TaskQueueManager {
   // tasks posted to the main loop. The batch size is 1 by default.
   void SetWorkBatchSize(int work_batch_size);
 
+  // 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);
+  void RemoveTaskObserver(base::MessageLoop::TaskObserver* task_observer);
+
   void SetTimeSourceForTesting(scoped_refptr<cc::TestNowSource> time_source);
 
  private:
@@ -141,9 +147,12 @@ class CONTENT_EXPORT TaskQueueManager {
   bool SelectWorkQueueToService(size_t* out_queue_index);
 
   // Runs a single nestable task from the work queue designated by
-  // |queue_index|. Non-nestable task are reposted on the run loop.
-  // The queue must not be empty.
-  void ProcessTaskFromWorkQueue(size_t queue_index);
+  // |queue_index|. If |has_previous_task| is true, |previous_task| should
+  // contain the previous task in this work batch. Non-nestable task are
+  // reposted on the run loop. The queue must not be empty.
+  void ProcessTaskFromWorkQueue(size_t queue_index,
+                                bool has_previous_task,
+                                base::PendingTask* previous_task);
 
   bool RunsTasksOnCurrentThread() const;
   bool PostDelayedTask(const tracked_objects::Location& from_here,
@@ -177,6 +186,8 @@ class CONTENT_EXPORT TaskQueueManager {
 
   scoped_refptr<cc::TestNowSource> time_source_;
 
+  ObserverList<base::MessageLoop::TaskObserver> task_observers_;
+
   base::WeakPtrFactory<TaskQueueManager> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(TaskQueueManager);