The set of all prototype shapes was a process-global static, which
meant that Shape::invalidate_all_prototype_chains_leading_to_this()
had to iterate over every prototype shape from every Realm in the
process.
This was catastrophic for pages that load many SVG-as-img resources,
since each SVG image creates its own Realm with a full set of JS
intrinsics and web prototypes. With N SVG images, each adding ~100
properties to their ObjectPrototype, this became O(N * 100 * M)
where M is the total number of prototype shapes across all Realms.
Since prototype chains never cross Realm boundaries, we can scope
the tracking to each Realm, making the invalidation cost independent
of the number of Realms in the process.
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