The new API with v8::BackingStore should be used instead as explained in
https://docs.google.com/document/d/1sTc_jRL87Fu175Holm5SV0kajkseGl2r8ifGY76G35k

This also relaxes the pre-condition for [Shared]ArrayBuffer::Detach to
not require externalization first.

Bug: v8:9380, v8:9908
Change-Id: Idd119fcd28be84a2fae74ae86f7381fd997766f5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1859628
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64625}