scheduler: Add support for tracing scheduler state

content/renderer/scheduler/task_queue_selector.h

 // Copyright 2014 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 CONTENT_RENDERER_SCHEDULER_TASK_QUEUE_SELECTOR_H_
 #define CONTENT_RENDERER_SCHEDULER_TASK_QUEUE_SELECTOR_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
 
 namespace base {
 class TaskQueue;
-}
+namespace debug {
+class TracedValue;
+}  // namespace debug
+}  // namespace base
 
 namespace content {
 
 class TaskQueueSelector {
  public:
   virtual ~TaskQueueSelector() {}
 
   // Called once to register the work queues to be selected from. This function
   // is called on the main thread.
   virtual void RegisterWorkQueues(
       const std::vector<const base::TaskQueue*>& work_queues) = 0;
 
   // Called to choose the work queue from which the next task should be taken
   // and run. Return true if |out_queue| indicates the queue to service or
   // false to avoid running any task.
   //
   // This function is called on the main thread.
   virtual bool SelectWorkQueueToService(size_t* out_queue_index) = 0;
+
+  // Serialize the selector state for tracing.
+  virtual void AsValueInto(base::debug::TracedValue* state) const = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_SCHEDULER_TASK_QUEUE_SELECTOR_H_