[ios] introduce weak_nsobject (#47947)

Introduce weak_nsobject from chromium. 

There are some usages of weak_ptr wrapping Objective-C ids, weak_ptr is not really designed for ids and such usages are blocking the arc migration. 

This PR mostly copies the weak_nsobject from chromium, at the same hash that we copied the ARC/MRC compatible scoped_nsobject: https://chromium.googlesource.com/chromium/src/+/fd625125b8a6a3aceaf09993a5f74cfe5368b17f

To match how we used weak_ptr for those ids, I made some changes to the weak_nsobject:
- WeakNSObjects needs to be generated by a WeakNSObjectFactory. The WeakNSObjectFactory is owned by the objc class and acts as the generator of the WeakNSObjects. All the WeakNSObjects' derefing thread should be the same of the WeakNSObjectFactory's creation thread.
- chromuim's WeakNSObjects can be detached from the thread and re-attached to a new thread. To match our weak_ptr behavior, I changed WeakNSObjects to be only accessed from a single thread, the same as weak_ptr

This PR also moves the FlutterEngine to use WeakNSObject and updated related classes.

part of https://github.com/flutter/flutter/issues/137801

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
19 files changed
tree: 8e66010dedbe75a62608ae08fbe26cddc3196621
  1. .github/
  2. assets/
  3. benchmarking/
  4. build/
  5. ci/
  6. common/
  7. display_list/
  8. docs/
  9. examples/
  10. flow/
  11. flutter_frontend_server/
  12. flutter_vma/
  13. fml/
  14. impeller/
  15. lib/
  16. runtime/
  17. shell/
  18. skia/
  19. sky/
  20. testing/
  21. third_party/
  22. tools/
  23. vulkan/
  24. wasm/
  25. web_sdk/
  26. .ci.yaml
  27. .clang-format
  28. .clang-tidy
  29. .gitattributes
  30. .gitignore
  31. .pylintrc
  32. .style.yapf
  33. analysis_options.yaml
  34. AUTHORS
  35. BUILD.gn
  36. CODEOWNERS
  37. CONTRIBUTING.md
  38. DEPS
  39. Doxyfile
  40. LICENSE
  41. README.md
README.md

Flutter Engine

OpenSSF Scorecard SLSA 1

Flutter is Google's SDK for crafting beautiful, fast user experiences for mobile, web, and desktop from a single codebase. Flutter works with existing code, is used by developers and organizations around the world, and is free and open source.

The Flutter Engine is a portable runtime for hosting Flutter applications. It implements Flutter's core libraries, including animation and graphics, file and network I/O, accessibility support, plugin architecture, and a Dart runtime and compile toolchain. Most developers will interact with Flutter via the Flutter Framework, which provides a modern, reactive framework, and a rich set of platform, layout and foundation widgets.

If you want to run/contribute to Flutter Web engine, more tooling can be found at felt. This is a tool written to make web engine development experience easy.

If you are new to Flutter, then you will find more general information on the Flutter project, including tutorials and samples, on our Web site at Flutter.dev. For specific information about Flutter's APIs, consider our API reference which can be found at the docs.flutter.dev.

Flutter is a fully open source project, and we welcome contributions. Information on how to get started can be found at our contributor guide.