Add const Page iterators to Spaces, and add whichever const methods are
necessary for this to work. This and a couple more const methods allows
us to make Heap::Contains const.

Change-Id: I1b63a10575ccdb8a3979aef4fa63a97b288ff836
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2198975
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67776}