The TaskCostEstimator uses the old base::MessageLoop::TaskObserver API
and it samples Now in WillProcessTask & DidProcessTask.

This seems wasteful since we have TaskTimeObservers.  Unfortunately
TaskTimeObservers are a SequenceManager level API but TaskCostEstimator
needs a TaskQueue level API.   Alternatively we could consider a map of
TaskQueue* to TaskCostEstimator held by MainThreadSchedulerImpl and
referenced by MainThreadSchedulerImpl::OnTaskStarted / OnTaskCompleted.