[Approach 3] Pre-allocate IncomigTaskQueue before MessageLoop for faster thread startup

base/message_loop/incoming_task_queue.h

Index: base/message_loop/incoming_task_queue.h
diff --git a/base/message_loop/incoming_task_queue.h b/base/message_loop/incoming_task_queue.h
index 72e1f30282966de72b01a505a6144839a1d00643..193ff7a160ae8e22e5f319024485dd2b84364729 100644
--- a/base/message_loop/incoming_task_queue.h
+++ b/base/message_loop/incoming_task_queue.h
@@ -6,6 +6,7 @@
 #define BASE_MESSAGE_LOOP_INCOMING_TASK_QUEUE_H_
 
 #include "base/base_export.h"
+#include "base/debug/task_annotator.h"
 #include "base/memory/ref_counted.h"
 #include "base/pending_task.h"
 #include "base/synchronization/lock.h"
@@ -24,7 +25,11 @@ namespace internal {
 class BASE_EXPORT IncomingTaskQueue
     : public RefCountedThreadSafe<IncomingTaskQueue> {
  public:
-  explicit IncomingTaskQueue(MessageLoop* message_loop);
+  IncomingTaskQueue();
+
+  // Associates the |message_loop| to this task queue and starts scheduling
+  // tasks.
+  void StartScheduling(MessageLoop* message_loop);
 
   // Appends a task to the incoming queue. Posting of all tasks is routed though
   // AddToIncomingQueue() or TryAddToIncomingQueue() to make sure that posting
@@ -53,6 +58,10 @@ class BASE_EXPORT IncomingTaskQueue
   // Disconnects |this| from the parent message loop.
   void WillDestroyCurrentMessageLoop();
 
+  // Returns the TaskAnnotator which is used to add debug information to posted
+  // tasks.
+  debug::TaskAnnotator* task_annotator() { return &task_annotator_; }
+
  private:
   friend class RefCountedThreadSafe<IncomingTaskQueue>;
   virtual ~IncomingTaskQueue();
@@ -86,11 +95,16 @@ class BASE_EXPORT IncomingTaskQueue
 
   // True if our message loop has already been scheduled and does not need to be
   // scheduled again until an empty reload occurs.
-  bool message_loop_scheduled_;
+  bool message_loop_scheduled_ = false;
+
+  // True after WillDestroyCurrentMessageLoop is called.
+  bool message_loop_terminated_ = false;
 
   // True if we always need to call ScheduleWork when receiving a new task, even
   // if the incoming queue was not empty.
-  const bool always_schedule_work_;
+  bool always_schedule_work_ = false;
+
+  debug::TaskAnnotator task_annotator_;
 
   DISALLOW_COPY_AND_ASSIGN(IncomingTaskQueue);
 };