We have many cases where we use CheckedUnsafePtr just to make sure that we get notified (by an assertion) that an object is being destroyed before it was removed from a global hash table or an array. Ordinary replacement of CheckedUnsafePtr with RefPtr would make it hard to find out where we miss an unregisteration in a global hash table/array (global tracking would keep the object alive forever). I think we can create a safer alternative to CheckedUnsafePtr which would preserve the checks/assertions for a missing unregistration.
This can lead to elimination of CheckedUnsafePtr for good.

This is a new version of D215351 which addresses some initial design comments.
A thread safe version of RefCountedSharedObject should be able to avoid a
static mutex and use std::atomic.
The new design is not limited to always trigger a diagnostic assertion. It
allows to do custom checks and cleanup when the holder becomes the last one
holding a strong ref to the object.

This is only a demo patch, not intended for landing.