New internal properties expose the byte length of an ArrayBuffer as well
as the pointer to the backing store, which will serve as a unique ID
to show when SharedArrayBuffers in different workers are the same buffer.

Bug: chromium:1163800
Change-Id: I49930765cb38f75ba5c6cee5a0a6827f4fec42d5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2618242
Commit-Queue: Eric Leese <leese@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72000}