Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/prioritizing_task_queue_selector.cc

Index: components/scheduler/child/prioritizing_task_queue_selector.cc
diff --git a/components/scheduler/child/prioritizing_task_queue_selector.cc b/components/scheduler/child/prioritizing_task_queue_selector.cc
index 5275f72b65d2eda7abe4aa77b4985082215cc07b..e0634629a15f218275174d046195cc2ff087329f 100644
--- a/components/scheduler/child/prioritizing_task_queue_selector.cc
+++ b/components/scheduler/child/prioritizing_task_queue_selector.cc
@@ -11,12 +11,17 @@
 namespace scheduler {
 
 PrioritizingTaskQueueSelector::PrioritizingTaskQueueSelector()
-    : starvation_count_(0), task_queue_selector_observer_(nullptr) {
+    : starvation_count_(0), task_queue_selector_observer_(nullptr),
+      pre_shutdown_(false) {
 }
 
 PrioritizingTaskQueueSelector::~PrioritizingTaskQueueSelector() {
 }
 
+void PrioritizingTaskQueueSelector::PreShutdown() {
+  pre_shutdown_ = true;
+}
+
 void PrioritizingTaskQueueSelector::RegisterWorkQueues(
     const std::vector<const base::TaskQueue*>& work_queues) {
   DCHECK(main_thread_checker_.CalledOnValidThread());
@@ -33,6 +38,8 @@ void PrioritizingTaskQueueSelector::RegisterWorkQueues(
 
 void PrioritizingTaskQueueSelector::SetQueuePriority(size_t queue_index,
                                                      QueuePriority priority) {
+  if (pre_shutdown_)
+    return;
   DCHECK(main_thread_checker_.CalledOnValidThread());
   DCHECK_LT(queue_index, work_queues_.size());
   DCHECK_LT(priority, QUEUE_PRIORITY_COUNT);
@@ -44,10 +51,14 @@ void PrioritizingTaskQueueSelector::SetQueuePriority(size_t queue_index,
 
 void PrioritizingTaskQueueSelector::EnableQueue(size_t queue_index,
                                                 QueuePriority priority) {
+  if (pre_shutdown_)
+    return;
   SetQueuePriority(queue_index, priority);
 }
 
 void PrioritizingTaskQueueSelector::DisableQueue(size_t queue_index) {
+  if (pre_shutdown_)
+    return;
   DisableQueueInternal(queue_index);
 }