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 TaskQueue::TaskIsOlderThanQueuedTasks(const base::PendingTask* task) {
  lock_.AssertAcquired();
  if (!task)
    return true;

  if (incoming_queue_.empty())
    return false;

  base::PendingTask oldest_queued_task = incoming_queue_.front();
  DCHECK(oldest_queued_task.delayed_run_time.is_null());
  DCHECK(task->delayed_run_time.is_null());

  return oldest_queued_task < *task;
}

bool TaskQueue::TaskIsOlderThanQueuedTasks(const base::PendingTask* task) {
  lock_.AssertAcquired();
  if (!task)
    return true;

  if (incoming_queue_.empty())
    return false;

  base::PendingTask oldest_queued_task = incoming_queue_.front();
  DCHECK(oldest_queued_task.delayed_run_time.is_null());
  DCHECK(task->delayed_run_time.is_null());

  return oldest_queued_task < *task;
}
