Shared read-only heap means that all isolates within a process must
share the same snapshot. Pass the back-end snapshot to the front-end
runner to fix that.

Bug: v8:7464
Change-Id: I0ec591a919d4d462ef38e372907592df3c759521
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1669691
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62349}