Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/prioritizing_task_queue_selector_unittest.cc

Index: components/scheduler/child/prioritizing_task_queue_selector_unittest.cc
diff --git a/components/scheduler/child/prioritizing_task_queue_selector_unittest.cc b/components/scheduler/child/prioritizing_task_queue_selector_unittest.cc
index 81a2a945e6fbbd8b1f474bcebfdad97c29e525bf..3ff62239a6edea4b172eb31f3b9081a69ac1be14 100644
--- a/components/scheduler/child/prioritizing_task_queue_selector_unittest.cc
+++ b/components/scheduler/child/prioritizing_task_queue_selector_unittest.cc
@@ -245,4 +245,25 @@ TEST_F(PrioritizingTaskQueueSelectorTest, TestBestEffortGetsStarved) {
   }
 }
 
+TEST_F(PrioritizingTaskQueueSelectorTest, PreShutdownBlocksDisable) {
+  selector_.SetQueuePriority(0, PrioritizingTaskQueueSelector::NORMAL_PRIORITY);
+  selector_.PreShutdown();
+  selector_.DisableQueue(0);
+  EXPECT_TRUE(selector_.IsQueueEnabled(0));
+}
+
+TEST_F(PrioritizingTaskQueueSelectorTest, PreShutdownBlocksSetQueuePriority) {
+  selector_.DisableQueue(0);
+  selector_.PreShutdown();
+  selector_.SetQueuePriority(0, PrioritizingTaskQueueSelector::NORMAL_PRIORITY);
+  EXPECT_FALSE(selector_.IsQueueEnabled(0));
+}
+
+TEST_F(PrioritizingTaskQueueSelectorTest, PreShutdownBlocksEnable) {
+  selector_.DisableQueue(0);
+  selector_.PreShutdown();
+  selector_.EnableQueue(0, PrioritizingTaskQueueSelector::NORMAL_PRIORITY);
+  EXPECT_FALSE(selector_.IsQueueEnabled(0));
+}
+
 }  // namespace scheduler