Change Heap::IterateStrongRoots to never iterate the read-only roots. In
doing so remove VISIT_ALL_BUT_READ_ONLY and
VISIT_ONLY_STRONG_FOR_SERIALIZATION. All such uses should now use
VISIT_ALL and VISIT_ONLY_STRONG. Where ReadOnlyRoots iteration is
required, this adds ReadOnlyRoots(isolate)->Iterate() at the call site.

Add new begin, end, strong_mutable_roots_begin and
strong_mutable_roots_end methods to RootsTable and try and make the
existing uses a little more consistent.

Bug: v8:8191
Change-Id: Ie9d0f9e5186db418428e2fafd38432b0bd879daa
Reviewed-on: https://chromium-review.googlesource.com/c/1278500
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56672}