This CL adds support for testing web snapshots through mjsunit tests.
To allow for taking and using web snapshots from JavaScript, two
methods, Realm.takeWebSnapshot() and Realm.useWebSnapshot(), are
introduced in d8.

Both of these methods accept a Realm as a parameter, allowing for
mjsunit tests to create and use the snapshot in different realms.

To return the snapshot data, Realm.takeWebSnapshot() creates and
returns a snapshot object with the snapshot data stored as an embedder
field.

Bug: v8:11525, v8:11706
Change-Id: I6e514e10eabf5bdb96d81e2697d4ddc49d92de73
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2905610Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Vicky Kontoura <vkont@google.com>
Cr-Commit-Position: refs/heads/master@{#74783}