Now that we no longer have ObjectPtr and Object* return types,
one way to load the slot's contents is enough.

Bug: v8:3770
Change-Id: I5acaeed22e68595b0e0ba036fcc4ac3d15c57462
Reviewed-on: https://chromium-review.googlesource.com/c/1400416
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58678}