Moves STRUCT_LIST AND ALLOCATION_SITE_LIST into roots.h and adds access
to their associated maps using ReadOnlyRoots.

Also corrects the location of external_map, message_object_map,
empty_script, many_closures_cell, invalid_prototype_validity_cell and
builtins_constants_table which are not in RO_SPACE.

Finally this adds a convenience ReadOnlyRoots(Isolate*) constructor.

Bug: v8:7786
Change-Id: I4982dd0cbea2062a124605678599ba48831f020f
Reviewed-on: https://chromium-review.googlesource.com/1124319Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54178}