Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/task_queue_manager.cc

Index: components/scheduler/child/task_queue_manager.cc
diff --git a/components/scheduler/child/task_queue_manager.cc b/components/scheduler/child/task_queue_manager.cc
index 2c03bd6a49eb20482767a09f061bd6971198a6c1..e4fdf36d962ea98ec509b908fc5083d6e711f2ab 100644
--- a/components/scheduler/child/task_queue_manager.cc
+++ b/components/scheduler/child/task_queue_manager.cc
@@ -549,6 +549,11 @@ void TaskQueueManager::PumpQueue(size_t queue_index) {
   queue->PumpQueue();
 }
 
+void TaskQueueManager::FlushQueue(size_t queue_index) {
+  internal::TaskQueue* queue = Queue(queue_index);
+  queue->WillDeleteTaskQueueManager();
+}
+
 bool TaskQueueManager::UpdateWorkQueues(
     const base::PendingTask* previous_task) {
   // TODO(skyostil): This is not efficient when the number of queues grows very