Avoid the SFI lookup in the script's weak SFI list, which could fail if
the function was somehow GCed between allocation and finalization, and
instead hold a Handle to each finalizing SFI in the finalization data
structure.

For similar reasons, also check that the compiled function to have its
compilation finalized is still compiled by that point, because it could
have been bytecode flushed.

Bug: chromium:1081691
Change-Id: I4e1f681263473a966e782b23291636d10a9209c5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2199349
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67802}