All of the places where MicrotasksScope is placed on the
stack are callbacks from JavaScript, so microtasks would
never run microtasks inside one of these methods.

Change-Id: I6dc36971189d6479138b58b90e3b9acae0c3ef73
Reviewed-on: https://chromium-review.googlesource.com/c/1277721Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56687}