Blink get's highly confused when we change the "entered or
microtask context" for MicrotaskCallbacks.

Bug: chromium:808911, v8:7253
Change-Id: Iee1e872b81a7cddd7138d22d10fa12aa71935dbf
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/903769Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51115}