When async stepping through code where only extension scripts are on the
stack, we the iterator is not gonna yield any frames, since it ignores
all extension frames.

Bug: chromium:1045863
Change-Id: Ifdb966e47bf1d090fa695d066003c7aff343f658
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2033167
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66069}