Simplify the blink scheduler's handling of high priority tasks, because
we don't want slow input tasks to starve out compositor tasks.

BUG=391005

Review URL: https://codereview.chromium.org/496323002

git-svn-id: svn://svn.chromium.org/blink/trunk@180787 bbb929c8-8fbe-4397-9dbb-9b2b20218538