(this is attempt 3; details below)
Remove obsolete object caches and introduce a simpler way to manage
native objects:
* Remove the unused `SynchronousSkiaObjectCache`.
* Introduce new library `native_memory.dart` that's smaller and simpler
than `skia_object_cache.dart`.
* Introduce two types of native object references:
* `UniqueRef` a reference with a unique Dart object owner.
* `CountedRef` a ref-counted reference with multiple Dart object owners.
* All native references use GC (via `FinalizationRegistry`) as a
back-up.
* The new library removes everything related to object resurrection that
was needed only in browsers that didn't support `FinalizationRegistry`.
All browsers support it now.
* Remove the ad hoc `SkParagraph` cache that predates the introduction
of `Paragraph.dispose`.
* Rewrite `CkParagraph` in terms of `UniqueRef`.
* Rewrite `CkImage` in terms of `CountedRef`; delete `SkiaObjectBox`.
This PR does not migrate all objects from the old
`skia_object_cache.dart` to `native_memory.dart`. That would be too big
of a change. The migration can be done in multiple smaller PRs.
This also removes a few unnecessary relayouts observed in
https://github.com/flutter/flutter/issues/120921, but not all of them
(more details in
https://github.com/flutter/flutter/issues/120921#issuecomment-1481958762)
## About attempt 3
More about [attempt 2
here](https://github.com/flutter/engine/pull/40862).
In this attempt 3 I'm replacing the `factory` with a top-level function.
ebfd37a6f7