HeapObjectIterator now has a DCHECK preventing iterating over read-only
space. This removes DumpMaps and switches to ReadOnlyHeapIterator
instead.

Bug: v8:7464
Change-Id: Ifd186d0e26d94c315308aa3a6ecb9da02a270fc1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1622120
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61708}