This is another step towards efficient external reference access from
off-heap builtins.

Bug: v8:6666
Change-Id: I6f128a8a64724acaa092f28b1fceae01e3c05487
Reviewed-on: https://chromium-review.googlesource.com/968481Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52039}