During marking, shared objects should not be marked when not
marking the shared heap (i.e. when not doing a shared GC).
Doing so is unsafe, as marking can race with sweeper threads
sweeping the shared heap. This CL adds the missing check on
weak object marking.

Bug: v8:12687
Change-Id: I1e0b8ba6b09bbcf665e5ff0f6242ed88f543c1fa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3583610Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79964}