Fix ThreadTaskRunnerHandle::Get calls during SequenceManager destruction

Some PostTask closures do exciting things on destruction like posting
other tasks. Some of them call ThreadTaskRunnerHandle::Get and these where
crashing because it had gone away.  This patch fixes that.

Bug: 933295
Change-Id: I7f48e2743dcebe38f9f6e9f08030d762760fc733
Reviewed-on: https://chromium-review.googlesource.com/c/1482954Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634649}

base/task/sequence_manager/sequence_manager_impl.cc

@@ -129,15 +129,15 @@ SequenceManagerImpl::~SequenceManagerImpl() {
   DCHECK(!controller_->GetBoundMessagePump() ||
          main_thread_only().task_execution_stack.empty());
 
-  // TODO(altimin): restore default task runner automatically when
-  // ThreadController is destroyed.
-  controller_->RestoreDefaultTaskRunner();
-
   for (internal::TaskQueueImpl* queue : main_thread_only().active_queues) {
     main_thread_only().selector.RemoveQueue(queue);
     queue->UnregisterTaskQueue();
   }
 
+  // TODO(altimin): restore default task runner automatically when
+  // ThreadController is destroyed.
+  controller_->RestoreDefaultTaskRunner();
+
   main_thread_only().active_queues.clear();
   main_thread_only().queues_to_gracefully_shutdown.clear();
   main_thread_only().selector.SetTaskQueueSelectorObserver(nullptr);

base/task/sequence_manager/sequence_manager_impl_unittest.cc

@@ -3920,6 +3920,21 @@ TEST_P(SequenceManagerTest, DestructorPostChainDuringShutdown) {
   EXPECT_TRUE(run);
 }
 
+TEST_P(SequenceManagerTest, DestructorPostsViaTaskRunnerHandleDuringShutdown) {
+  scoped_refptr<TestTaskQueue> task_queue = CreateTaskQueue();
+  bool run = false;
+  task_queue->task_runner()->PostTask(
+      FROM_HERE, RunOnDestruction(BindLambdaForTesting([&]() {
+        ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE,
+                                                base::BindOnce(&NopTask));
+        run = true;
+      })));
+
+  // Should not DCHECK when ThreadTaskRunnerHandle::Get() is invoked.
+  DestroySequenceManager();
+  EXPECT_TRUE(run);
+}
+
 TEST_P(SequenceManagerTest, CreateUnboundSequenceManagerWhichIsNeverBound) {
   // This should not crash.
   CreateUnboundSequenceManager();