Stack scan during marking for shared heap broke in
  https://crrev.com/c/3703837

This CL re-adds the client Isolate handling which is necessary as
those client Isolates may refer to the shared Isolate from stack.

Bug: v8:13019
Change-Id: I1ee27fb8bab173087a98a0b79f4126612427b016
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3736444Reviewed-by: Nikolaos Papaspyrou <nikolaos@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81460}