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}
internal::TaskQueue* TaskQueueManager::Queue(size_t queue_index) const {
  DCHECK_LT(queue_index, queues_.size());
  return queues_[queue_index].get();
}

internal::TaskQueue* TaskQueueManager::Queue(size_t queue_index) const {
  DCHECK_LT(queue_index, queues_.size());
  return queues_[queue_index].get();
}
