Isolate::UseAsSharedIsolate() was invoked after the Isolate was already
created. I think it is cleaner to have the shared-flag right when
constructing an Isolate. This way we can use that property already
when setting up the isolate.

Bug: v8:11708
Change-Id: Ibbfee09122b7b0361a5af7a1b559796594834813
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2885041Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74495}