Adds new VisitModes VISIT_ALL_BUT_READ_ONLY and
VISIT_STRONG_FOR_SERIALIZATION.

GC-related methods like MarkReachableObjects now now use
VISIT_ALL_BUT_READ_ONLY instead of VISIT_ALL. All GC-related VisitModes
skip iterating over the read-only roots.

All Serializer methods should always use a _FOR_SERIALIZATION value to
ensure they do visit the read-only roots.

Also adds RootsTable::read_only_roots_begin and end methods.

Bug: v8:7464
Change-Id: I468d7ae9f345d9fc0e10837f01dc5b92bd996412
Reviewed-on: https://chromium-review.googlesource.com/c/1256245Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56368}