Observer that increases frequency of checking if we reached
marking limit. Works only with --stress-marking.

Bug: v8:6972
Change-Id: I13544fdd8bb33738d78adbac96feb70222b5b634
Reviewed-on: https://chromium-review.googlesource.com/802434
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49907}