Keeps fml::scoped_* wrappers used in C++ class declarations in headers, since those are injected into both ARC and non-ARC translation units and these classes are ARC-safe. Once we've migrated all users of those headers to ARC, we can use the underlying classes directly.
No semantic changes, therefore not changes to tests.
Issue: https://github.com/flutter/flutter/issues/137801
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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