[Web] Update modifier state when Meta key is seen as Process key (#50779)

## Description

On Web, browsers can emit key events with a logical key sets to `Process` when the physical key is MetaLeft. Because the modifier state is 0 despite Meta key being pressed this will trigger an assert.
This PR adds some logic for this specific case. Maybe a more slightly broader solution will be needed (using the same logic for all modifiers ?). I focused on MetaLeft because it was directly reported on  https://github.com/flutter/flutter/issues/141186.

## Related Issue

Fixes https://github.com/flutter/flutter/issues/141186.

## Tests

Adds 1 test.
2 files changed
tree: acf5b692ea4c7e0b89e6f11cb2c5bf5fcb27ae6e
  1. .github/
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  3. benchmarking/
  4. bin/
  5. build/
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  7. common/
  8. display_list/
  9. docs/
  10. examples/
  11. flow/
  12. flutter_frontend_server/
  13. flutter_vma/
  14. fml/
  15. impeller/
  16. lib/
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  19. skia/
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  21. testing/
  22. third_party/
  23. tools/
  24. vulkan/
  25. wasm/
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  27. .ci.yaml
  28. .clang-format
  29. .clang-tidy
  30. .gitattributes
  31. .gitignore
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  34. analysis_options.yaml
  35. AUTHORS
  36. BUILD.gn
  37. CODEOWNERS
  38. CONTRIBUTING.md
  39. DEPS
  40. Doxyfile
  41. LICENSE
  42. README.md
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