Fix crash due to OnQueueNextWakeUpChanged firing after queue gone

This wasn't previously possible but the timing of the callback
was changed by patch
https://chromium-review.googlesource.com/c/chromium/src/+/1317629

Bug: 902277
Change-Id: I351872539b4ff2ebcb944a6ea76bd2021249f8ae
Reviewed-on: https://chromium-review.googlesource.com/c/1319613Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605703}

base/task/sequence_manager/sequence_manager_impl_unittest.cc

@@ -3064,6 +3064,20 @@ TEST_P(SequenceManagerTest, ProcessTasksWithTaskTimeObservers) {
   UnsetOnTaskHandlers(runners_[0]);
 }
 
+TEST_P(SequenceManagerTest, ObserverNotFiredAfterTaskQueueDestructed) {
+  scoped_refptr<TestTaskQueue> main_tq = CreateTaskQueue();
+
+  MockTaskQueueObserver observer;
+  main_tq->SetObserver(&observer);
+
+  // We don't expect the observer to fire if the TaskQueue gets destructed.
+  EXPECT_CALL(observer, OnQueueNextWakeUpChanged(_, _)).Times(0);
+  main_tq->PostTask(FROM_HERE, BindOnce(&NopTask));
+
+  main_tq = nullptr;
+  test_task_runner_->FastForwardUntilNoTasksRemain();
+}
+
 TEST_P(SequenceManagerTest, GracefulShutdown) {
   std::vector<TimeTicks> run_times;
   scoped_refptr<TestTaskQueue> main_tq = CreateTaskQueue();

base/task/sequence_manager/task_queue.cc

@@ -49,6 +49,7 @@ TaskQueue::~TaskQueue() {
 
   // If we've not been unregistered then this must occur on the main thread.
   DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
+  impl_->SetOnNextWakeUpChangedCallback(RepeatingCallback<void(TimeTicks)>());
   impl_->sequence_manager()->ShutdownTaskQueueGracefully(TakeTaskQueueImpl());
 }