a39108c5a4 (#47244) replaced gin_helper::EmitEvent with a direct
`v8::Function::Call()` in `WebWorkerObserver::ContextWillDestroy`
to avoid re-entering the microtask checkpoint during worker teardown.
V8 `DCHECK()`s that a policy is set. Under the old code path, this
happened with a node::CallbackScope. Under the new code path, it's
possible for a policy to not be set, causing that `DCHECK()` to fail.
This PR copies a39108c5a4's changes in `ShareEnvironmentWithContext()`:
it explicitly adds a `kDoNotRunMicrotasks` scope.
6c49cb3b27