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

base/message_loop/message_loop.h

Index: base/message_loop/message_loop.h
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index fd7596a79204ed89291f32b2f9135108a73b2882..e7228a7b31ec94b3a25c65a6aa676a0ed514b304 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -11,7 +11,6 @@
 #include "base/base_export.h"
 #include "base/basictypes.h"
 #include "base/callback_forward.h"
-#include "base/debug/task_annotator.h"
 #include "base/location.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
@@ -109,12 +108,36 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
 #endif  // defined(OS_ANDROID)
   };
 
+  struct BASE_EXPORT Options {
+    using MessagePumpFactory = Callback<scoped_ptr<MessagePump>()>;
+    Options();
+    ~Options();
+
+    // Specify the type of message loop.
+    // This is ignored if message_pump_factory.is_null() is false.
+    MessageLoop::Type message_loop_type;
+
+    // Used to create the MessagePump for the MessageLoop.
+    // If message_pump_factory.is_null() is true, then a MessagePump appropriate
+    // for |message_loop_type| is created. Setting this forces the
+    // MessageLoop::Type to TYPE_CUSTOM.
+    MessagePumpFactory message_pump_factory;
+
+    // If non-null incoming_task_queue is specified this message loop will
+    // use it as its incoming task queue.
+    scoped_refptr<internal::IncomingTaskQueue> incoming_task_queue;
+  };
+
   // Normally, it is not necessary to instantiate a MessageLoop.  Instead, it
   // is typical to make use of the current thread's MessageLoop instance.
+  explicit MessageLoop(const Options& options);
+
+  // Creates a MessageLoop for the supplied type. This constructor is used
+  // only for testing.
   explicit MessageLoop(Type type = TYPE_DEFAULT);
   // Creates a TYPE_CUSTOM MessageLoop with the supplied MessagePump, which must
-  // be non-NULL.
-  explicit MessageLoop(scoped_ptr<base::MessagePump> pump);
+  // be non-NULL. This constructor is used only for testing.
+  explicit MessageLoop(scoped_ptr<MessagePump> pump);
   ~MessageLoop() override;
 
   // Returns the MessageLoop object for the current thread, or null if none.
@@ -400,7 +423,9 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
 
   // Returns the TaskAnnotator which is used to add debug information to posted
   // tasks.
-  debug::TaskAnnotator* task_annotator() { return &task_annotator_; }
+  debug::TaskAnnotator* task_annotator() {
+    return incoming_task_queue_->task_annotator();
+  }
 
   // Runs the specified PendingTask.
   void RunTask(const PendingTask& pending_task);
@@ -412,7 +437,7 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
  private:
   friend class RunLoop;
 
-  // Configures various members for the two constructors.
+  // Configures various members for the constructors.
   void Init();
 
   // Invokes the actual run loop using the message pump.
@@ -498,8 +523,6 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
 
   ObserverList<TaskObserver> task_observers_;
 
-  debug::TaskAnnotator task_annotator_;
-
   scoped_refptr<internal::IncomingTaskQueue> incoming_task_queue_;
 
   // The message loop proxy associated with this message loop.