When performing a shared GC, we need to find references from the client
heaps into the shared heaps. For now we achieve this by simply
iterating all objects in client heaps.

We need to do this both for marking and when updating pointers after
evacuation.

Bug: v8:11708
Change-Id: Ic1dd94cc352be0404095e548979c37b1ef25682a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3300142
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78240}