ReadOnlySpaceObjectIterator did not iterate through objects, because of
a bug in the initialization of `cur_addr_` and `cur_end_`. This CL also
merges methods `Next` and `FromCurrentPage`.

Change-Id: Id56bff279216c7a5982d984d80b649bd0c915959
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3758225Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81668}