[web] remove obsolete object caches; simplify native object management

[yjbanov]

(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 flutter/flutter#120921, but not all of them (more details in flutter/flutter#120921 (comment))

About attempt 3

More about attempt 2 here.

In this attempt 3 I'm replacing the factory with a top-level function.

Benchmarks

Now that this landed in flutter/flutter I have some benchmark numbers from the devicelab. The text_out_of_picture_bounds benchmark dropped by 3-4x (lower is better):

The repro provided in flutter/flutter#123204 dropped from 110ms/frame to 10ms/frame.

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Changes reported for pull request #40894 at sha da3318d

[mdebbar]

The flutter-gold check is stuck due to a bug where Skia Gold gets confused when you force-push to a PR. In order to work around it, please push a new commit (not force-push).

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Golden file changes are available for triage from new commit, Click here to view.

Changes reported for pull request #40894 at sha 2cd7e56

[CaseyHillers]

LGTM on the Google Testing side