Fix bug with TaskQueueSelector and blocked queues

components/scheduler/base/task_queue_manager.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/scheduler/base/task_queue_manager.h"
 
 #include <queue>
 #include <set>
 
 #include "base/bind.h"
@@ skipping 177 matching lines @@
   // pump-after-wakeup queue.
   UpdateWorkQueues(false, nullptr);
 
   internal::TaskQueueImpl::Task previous_task;
   for (int i = 0; i < work_batch_size_; i++) {
     internal::WorkQueue* work_queue;
     if (!SelectWorkQueueToService(&work_queue)) {
       break;
     }
 
+    bool should_trigger_wakeup = work_queue->task_queue()->wakeup_policy() ==
+                                 TaskQueue::WakeupPolicy::CAN_WAKE_OTHER_QUEUES;
     switch (ProcessTaskFromWorkQueue(work_queue, &previous_task)) {
       case ProcessTaskResult::DEFERRED:
         // If a task was deferred, try again with another task. Note that this
         // means deferred tasks (i.e. non-nestable tasks) will never trigger
         // queue wake-ups.
         continue;
       case ProcessTaskResult::EXECUTED:
         break;
       case ProcessTaskResult::TASK_QUEUE_MANAGER_DELETED:
         return;  // The TaskQueueManager got deleted, we must bail out.
     }
-    bool should_trigger_wakeup = work_queue->task_queue()->wakeup_policy() ==
-                                 TaskQueue::WakeupPolicy::CAN_WAKE_OTHER_QUEUES;
+    work_queue = nullptr; // The queue may have been unregistered.
+
     UpdateWorkQueues(should_trigger_wakeup, &previous_task);
 
     // Only run a single task per batch in nested run loops so that we can
     // properly exit the nested loop when someone calls RunLoop::Quit().
     if (delegate_->IsNested())
       break;
   }
 
   // TODO(alexclarke): Consider refactoring the above loop to terminate only
   // when there's no more work left to be done, rather than posting a
@@ skipping 183 matching lines @@
     internal::WorkQueue* work_queue) {
   DCHECK(main_thread_checker_.CalledOnValidThread());
   DCHECK(!work_queue->Empty());
   if (observer_) {
     observer_->OnTriedToExecuteBlockedTask(*work_queue->task_queue(),
                                            *work_queue->GetFrontTask());
   }
 }
 
 }  // namespace scheduler