We want to be stricter about checking in the future, so give embedders a
way to disable checking while they fix their microtasks scopes.

BUG=chromium:728583
R=machenbach@chromium.org

Change-Id: I443575bf6820b432def59cbbd4d048b2007573c8
Reviewed-on: https://chromium-review.googlesource.com/522604
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45677}