Make IdleTimeEstimator ignore nested tasks

Unfortunately you can run a modal dialog inside a RAF so nesting of
compositor tasks is possible.  Like other places in the scheduler we
ignore nested tasks for timing computations.

BUG=543894

Review URL: https://codereview.chromium.org/1409073002

Cr-Commit-Position: refs/heads/master@{#354505}
3 files changed