Track list of all local heaps in the Safepoint class instead of the
Heap.

Bug: v8:10315
Change-Id: I1a1c847502ab5e8f368d4cc12d3cbaf3672af7cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2106197Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66745}