We assume that during bootstrapping, we won't create script contexts.
This is wrong, since JavaScript code in extensions may introduce
let/const variables.

R=jgruber@chromium.org

Change-Id: I02595abdbb65f41faffc90bde142849bbde6b554
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1666994
Auto-Submit: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63161}