Adopt a less severe anti-starvation policy for immediate tasks

While addressing starvation of non-delayed tasks by delayed tasks is
important, the strict round robin was causing lots of existing layout
tests to flake. A less extreme anti-starvation policy is still
effective but doesn't entail fixing loads of tests.

BUG=567618, 67039, 560817, 567884

Review URL: https://codereview.chromium.org/1507093004

Cr-Commit-Position: refs/heads/master@{#364106}
5 files changed