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

base/message_loop/incoming_task_queue.cc

Index: base/message_loop/incoming_task_queue.cc
diff --git a/base/message_loop/incoming_task_queue.cc b/base/message_loop/incoming_task_queue.cc
index c1ce939b0c4708ac431d80b54a35ee8ef73b314c..2e55fd6d33088b2e1b6e8e47207b90804e2447da 100644
--- a/base/message_loop/incoming_task_queue.cc
+++ b/base/message_loop/incoming_task_queue.cc
@@ -39,12 +39,21 @@ bool AlwaysNotifyPump(MessageLoop::Type type) {
 
 }  // namespace
 
-IncomingTaskQueue::IncomingTaskQueue(MessageLoop* message_loop)
+IncomingTaskQueue::IncomingTaskQueue()
     : high_res_task_count_(0),
-      message_loop_(message_loop),
-      next_sequence_num_(0),
-      message_loop_scheduled_(false),
-      always_schedule_work_(AlwaysNotifyPump(message_loop_->type())) {
+      message_loop_(nullptr),
+      next_sequence_num_(0) {
+}
+
+void IncomingTaskQueue::StartScheduling(MessageLoop* message_loop) {
+  AutoLock lock(incoming_queue_lock_);
+  DCHECK(!message_loop_);
+  DCHECK(!message_loop_terminated_);
+  DCHECK(message_loop);
+  message_loop_ = message_loop;
+  always_schedule_work_ = AlwaysNotifyPump(message_loop_->type());
+  if (!incoming_queue_.empty())
+    message_loop_->ScheduleWork();
 }
 
 bool IncomingTaskQueue::AddToIncomingQueue(
@@ -107,11 +116,13 @@ int IncomingTaskQueue::ReloadWorkQueue(TaskQueue* work_queue) {
 void IncomingTaskQueue::WillDestroyCurrentMessageLoop() {
   AutoLock lock(incoming_queue_lock_);
   message_loop_ = NULL;
+  message_loop_terminated_ = true;
 }
 
 IncomingTaskQueue::~IncomingTaskQueue() {
   // Verify that WillDestroyCurrentMessageLoop() has been called.
   DCHECK(!message_loop_);
+  DCHECK(message_loop_terminated_);
 }
 
 TimeTicks IncomingTaskQueue::CalculateDelayedRuntime(TimeDelta delay) {
@@ -131,7 +142,7 @@ bool IncomingTaskQueue::PostPendingTask(PendingTask* pending_task) {
   // This should only be called while the lock is taken.
   incoming_queue_lock_.AssertAcquired();
 
-  if (!message_loop_) {
+  if (message_loop_terminated_) {
     pending_task->task.Reset();
     return false;
   }
@@ -141,14 +152,14 @@ bool IncomingTaskQueue::PostPendingTask(PendingTask* pending_task) {
   // delayed_run_time value) and for identifying the task in about:tracing.
   pending_task->sequence_num = next_sequence_num_++;
 
-  message_loop_->task_annotator()->DidQueueTask("MessageLoop::PostTask",
-                                                *pending_task);
+  task_annotator_.DidQueueTask("MessageLoop::PostTask", *pending_task);
 
   bool was_empty = incoming_queue_.empty();
   incoming_queue_.push(*pending_task);
   pending_task->task.Reset();
 
-  if (always_schedule_work_ || (!message_loop_scheduled_ && was_empty)) {
+  if (message_loop_ &&
+      (always_schedule_work_ || (!message_loop_scheduled_ && was_empty))) {
     // Wake up the message loop.
     message_loop_->ScheduleWork();
     // After we've scheduled the message loop, we do not need to do so again