Adds an paged space object iterator for OffThreadSpace, and removes
the Heap pointer from the iterator so that it can be used off-thread.

Bug: chromium:1011762
Change-Id: I025edf144f393e61d89cce2485c0ff1fe8c80c56
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1991488
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65658}