When the embedder uses microtask scopes to control microtask execution,
we check in the V8 API that there's always a microtask scope on the
stack when entering V8. However, a microtask might call into the
embedder which in turn can call into V8, so we have to put a fake scope
on the stack before executing microtasks to avoid hitting the check.

Bug: chromium:728583
Change-Id: I99bcbcca26eaaac827d0ec09064044f616fb9efc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2853597
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74248}