This changes the default scope info for SyntheticFunctionContexts
to the EmptyScopeInfo which does not contain an extension slot.

The bug happened because, previously, the native context scope info
was used as dummy.

Change-Id: I4d6bf6918c11c79201d16bde99ed76800ad6f6c5
Bug: v8:10629
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2277806
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68636}