Allow a V8 step after doing an embedder step if there's time left. This
will speed up marking in certain situations.

Bug: chromium:909788, chromium:911053, chromium:843903
Change-Id: Ie15c30d5ce8443caf7217a834a120a19d0bb292b
Reviewed-on: https://chromium-review.googlesource.com/c/1367455
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58094}