This enables downloading web snapshots with XMLHttpRequest and
deserializing them.

Bug: v8:11525
Change-Id: I498f1e99795d474a1715fce9aa1d8c1a34651c42
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3585961Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80064}