Scheduler/child/TimeSource could be replaced with base/time/DefaultTickClock.

They both are totally same and TimeSource is removed.

BUG=494892
R=skyostil@chromium.org, mithro@mithis.com

Review URL: https://codereview.chromium.org/1163143002

Cr-Commit-Position: refs/heads/master@{#333035}
void TaskQueue::WillDeleteTaskQueueManager() {
  base::AutoLock lock(lock_);
  task_queue_manager_ = nullptr;
  delayed_task_queue_ = base::DelayedTaskQueue();
  incoming_queue_ = base::TaskQueue();
  work_queue_ = base::TaskQueue();
}

void TaskQueue::WillDeleteTaskQueueManager() {
  base::AutoLock lock(lock_);
  task_queue_manager_ = nullptr;
  delayed_task_queue_ = base::DelayedTaskQueue();
  incoming_queue_ = base::TaskQueue();
  work_queue_ = base::TaskQueue();
}
