This CL also makes existence of the optional padding field in JSArrayBuffer
explicit and ensures that the field stays cleared after initialization.

Bug: v8:8477, v8:8238
Change-Id: Ic4c5f6b0066903651f15bea91fbfe32ba62fa0e6
Reviewed-on: https://chromium-review.googlesource.com/c/1347469
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57750}