WeakFixedArrays can also contain strong pointers.

(Separating this fix from
https://chromium-review.googlesource.com/c/v8/v8/+/1075053 ; after that CL tests
will fail without this fix.)

BUG=v8:7308

Change-Id: I0d47ab179625bcbf6149cf02ad696f8af250ae8d
Reviewed-on: https://chromium-review.googlesource.com/1075270Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53386}