Raw data access is already possible via GetBackingStore()->GetData().
This API exposes a more efficient way for accessing
JSArrayBuffer::backing_store (which, despite the confusing name, is no
the BackingStore but its raw data pointer).

Bug: v8:10343
Change-Id: I695cea91e2c3de75ce6c86bac6e413ce6617958b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3764341Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81745}