If the incremental marker discovers more marking work and the concurrent
marking tasks have already exited, then new concurrent marking tasks
are scheduled to help the main thread marker.

BUG=chromium:694255

Change-Id: I559af62790e6034b23a412d3308ba8b6ae3d27c9
Reviewed-on: https://chromium-review.googlesource.com/574170Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46703}