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 19e8d2b65120525765527af4775d9564d2590f51..4e9910ae6b52af3b1259237ac8e266896da3cecf 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"
@@ -34,6 +35,9 @@ 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),
       quiescence_monitored_task_queue_mask_(
           ((1ull << total_task_queue_count) - 1ull) &
           ~(1ull << QueueId::IDLE_TASK_QUEUE) &
@@ -97,6 +101,19 @@ SchedulerHelper::SchedulerHelperDelegate::SchedulerHelperDelegate() {
 SchedulerHelper::SchedulerHelperDelegate::~SchedulerHelperDelegate() {
 }
 
+void SchedulerHelper::PreShutdown() {
+  CheckOnValidThread();
+  TRACE_EVENT0(disabled_by_default_tracing_category_, "PreShutdownInternal");
+  task_queue_selector_->DisableQueue(QueueId::CONTROL_TASK_AFTER_WAKEUP_QUEUE);
+  task_queue_selector_->DisableQueue(QueueId::DEFAULT_TASK_QUEUE);
+  task_queue_selector_->DisableQueue(QueueId::IDLE_TASK_QUEUE);
+  for (size_t i = TASK_QUEUE_COUNT; i < total_task_queue_count_; i++) {
+    task_queue_selector_->DisableQueue(i);
+  }
+  // Prevent any further enabling/disabling of queues.
+  task_queue_selector_->PreShutdown();
+}
+
 void SchedulerHelper::Shutdown() {
   CheckOnValidThread();
   task_queue_manager_.reset();
@@ -114,6 +131,11 @@ 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_;
 }
@@ -364,6 +386,8 @@ 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: