TestMetalContext: Use ARC-managed Metal types (flutter/engine#56717)
Previously, we could not include any Objective-C types in test_metal_context.h, since that file was transitively included in pure C++ translation units. All users have been refactored into backend-specific files, and all Metal-related files are Objective-C++ files.
We now use Metal types directly in the header, without the workarounds.
Issue: https://github.com/flutter/flutter/issues/158998
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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