BS for "backing store", obviously.

R=ahaas@chromium.org

Bug: v8:12425
Change-Id: I310ba8e34e8ca2c7b7bdeec5f1bc184330fbaf28
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3641175Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80477}