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}
bool TaskQueueManager::IsQueueEmpty(size_t queue_index) const {
  return Queue(queue_index)->GetQueueState() == QueueState::EMPTY;
}

bool TaskQueueManager::IsQueueEmpty(size_t queue_index) const {
  return Queue(queue_index)->GetQueueState() == QueueState::EMPTY;
}
