Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/scheduler_helper.cc

Index: components/scheduler/child/scheduler_helper.cc
diff --git a/components/scheduler/child/scheduler_helper.cc b/components/scheduler/child/scheduler_helper.cc
index cb2d191ee2860f15286c881df60117ac61a86821..f0f80e9c8504f034d2c20266f931ec07dc0e4665 100644
--- a/components/scheduler/child/scheduler_helper.cc
+++ b/components/scheduler/child/scheduler_helper.cc
@@ -4,6 +4,7 @@
 
 #include "components/scheduler/child/scheduler_helper.h"
 
+#include "base/synchronization/waitable_event.h"
 #include "base/trace_event/trace_event.h"
 #include "base/trace_event/trace_event_argument.h"
 #include "components/scheduler/child/nestable_single_thread_task_runner.h"
@@ -33,6 +34,10 @@ SchedulerHelper::SchedulerHelper(
           QueueId::CONTROL_TASK_AFTER_WAKEUP_QUEUE)),
       default_task_runner_(
           task_queue_manager_->TaskRunnerForQueue(QueueId::DEFAULT_TASK_QUEUE)),
+      shutdown_task_runner_(task_queue_manager_->TaskRunnerForQueue(
+          QueueId::SHUTDOWN_TASK_QUEUE)),
+      total_task_queue_count_(total_task_queue_count),
+      in_preshutdown_(false),
       quiescence_monitored_task_queue_mask_(
           ((1ull << total_task_queue_count) - 1ull) &
           ~(1ull << QueueId::IDLE_TASK_QUEUE) &
@@ -63,18 +68,16 @@ SchedulerHelper::SchedulerHelper(
                  weak_scheduler_ptr_),
       tracing_category));
 
-  task_queue_selector_->SetQueuePriority(
-      QueueId::CONTROL_TASK_QUEUE,
-      PrioritizingTaskQueueSelector::CONTROL_PRIORITY);
+  SetQueuePriority(QueueId::CONTROL_TASK_QUEUE,
+                   PrioritizingTaskQueueSelector::CONTROL_PRIORITY);
 
-  task_queue_selector_->SetQueuePriority(
-      QueueId::CONTROL_TASK_AFTER_WAKEUP_QUEUE,
-      PrioritizingTaskQueueSelector::CONTROL_PRIORITY);
+  SetQueuePriority(QueueId::CONTROL_TASK_AFTER_WAKEUP_QUEUE,
+                   PrioritizingTaskQueueSelector::CONTROL_PRIORITY);
   task_queue_manager_->SetPumpPolicy(
       QueueId::CONTROL_TASK_AFTER_WAKEUP_QUEUE,
       TaskQueueManager::PumpPolicy::AFTER_WAKEUP);
 
-  task_queue_selector_->DisableQueue(QueueId::IDLE_TASK_QUEUE);
+  DisableQueue(QueueId::IDLE_TASK_QUEUE);
   task_queue_manager_->SetPumpPolicy(QueueId::IDLE_TASK_QUEUE,
                                      TaskQueueManager::PumpPolicy::MANUAL);
 
@@ -96,6 +99,20 @@ SchedulerHelper::SchedulerHelperDelegate::SchedulerHelperDelegate() {
 SchedulerHelper::SchedulerHelperDelegate::~SchedulerHelperDelegate() {
 }
 
+void SchedulerHelper::PreShutdown() {
+  CheckOnValidThread();
+  DCHECK(!in_preshutdown_);
+  TRACE_EVENT0(disabled_by_default_tracing_category_, "PreShutdown");
+  // Disable everything except the shutdown task queue.
+  for (size_t i = 0; i < total_task_queue_count_; i++) {
+    if (i == SHUTDOWN_TASK_QUEUE)
+      continue;
+    DisableQueue(i);
+  }
+  // Ensure that the queues don't get re-enabled.
+  in_preshutdown_ = true;
+}
+
 void SchedulerHelper::Shutdown() {
   CheckOnValidThread();
   task_queue_manager_.reset();
@@ -113,6 +130,11 @@ scoped_refptr<SingleThreadIdleTaskRunner> SchedulerHelper::IdleTaskRunner() {
 }
 
 scoped_refptr<base::SingleThreadTaskRunner>
+SchedulerHelper::ShutdownTaskRunner() {
+  return shutdown_task_runner_;
+}
+
+scoped_refptr<base::SingleThreadTaskRunner>
 SchedulerHelper::ControlTaskRunner() {
   return control_task_runner_;
 }
@@ -254,9 +276,8 @@ void SchedulerHelper::StartIdlePeriod(IdlePeriodState new_state,
   CheckOnValidThread();
   DCHECK(IsInIdlePeriod(new_state));
 
-  task_queue_selector_->EnableQueue(
-      QueueId::IDLE_TASK_QUEUE,
-      PrioritizingTaskQueueSelector::BEST_EFFORT_PRIORITY);
+  EnableQueue(QueueId::IDLE_TASK_QUEUE,
+              PrioritizingTaskQueueSelector::BEST_EFFORT_PRIORITY);
   task_queue_manager_->PumpQueue(QueueId::IDLE_TASK_QUEUE);
   idle_period_state_ = new_state;
 
@@ -293,7 +314,7 @@ void SchedulerHelper::EndIdlePeriod() {
     TRACE_EVENT_ASYNC_END0(tracing_category_, idle_period_tracing_name_, this);
   }
 
-  task_queue_selector_->DisableQueue(QueueId::IDLE_TASK_QUEUE);
+  DisableQueue(QueueId::IDLE_TASK_QUEUE);
   idle_period_state_ = IdlePeriodState::NOT_IN_IDLE_PERIOD;
   idle_period_deadline_ = base::TimeTicks();
 }
@@ -355,6 +376,8 @@ void SchedulerHelper::SetQueuePriority(
     size_t queue_index,
     PrioritizingTaskQueueSelector::QueuePriority priority) {
   CheckOnValidThread();
+  if (in_preshutdown_)
+    return;
   return task_queue_selector_->SetQueuePriority(queue_index, priority);
 }
 
@@ -362,11 +385,15 @@ void SchedulerHelper::EnableQueue(
     size_t queue_index,
     PrioritizingTaskQueueSelector::QueuePriority priority) {
   CheckOnValidThread();
+  if (in_preshutdown_)
+    return;
   task_queue_selector_->EnableQueue(queue_index, priority);
 }
 
 void SchedulerHelper::DisableQueue(size_t queue_index) {
   CheckOnValidThread();
+  if (in_preshutdown_)
+    return;
   task_queue_selector_->DisableQueue(queue_index);
 }
 
@@ -382,6 +409,8 @@ const char* SchedulerHelper::TaskQueueIdToString(QueueId queue_id) {
       return "default_tq";
     case IDLE_TASK_QUEUE:
       return "idle_tq";
+    case SHUTDOWN_TASK_QUEUE:
+      return "shutdown_tq";
     case CONTROL_TASK_QUEUE:
       return "control_tq";
     case CONTROL_TASK_AFTER_WAKEUP_QUEUE: