The main thread now can reduce marking step size if concurrent marking
tasks are making progress and the bailout worklist is empty.

Bug: chromium:694255
Change-Id: I2f58530f184c03667ab3a170a1f6309929645c7c
Reviewed-on: https://chromium-review.googlesource.com/735859
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49671}