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::PostDelayedTask(
    const tracked_objects::Location& from_here,
    const base::Closure& task,
    base::TimeDelta delay) {
  DCHECK(delay > base::TimeDelta());
  return main_task_runner_->PostDelayedTask(from_here, task, delay);
}

bool TaskQueueManager::PostDelayedTask(
    const tracked_objects::Location& from_here,
    const base::Closure& task,
    base::TimeDelta delay) {
  DCHECK(delay > base::TimeDelta());
  return main_task_runner_->PostDelayedTask(from_here, task, delay);
}
