The read_only_object_cache call cannot return a vector when it is backed
by the embedded heap, so this adds a few methods to abstract this away.

ExtendReadOnlyObjectCache will eventually have a check to reject adding
objects to the embedded read-only object cache.

Prior to this change the read-only object cache would only be extended
conditionally if needed. Since it started out empty it was always
extended, so this removes this logic.

Bug: v8:7464
Change-Id: I5b172f629ac48be5cbb8f78b03a0a213ebd570e9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1619745Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Cr-Commit-Position: refs/heads/master@{#61661}