Moves code specific to each graphics backend into the (existing) translation unit associated with that backend.
Previously, we could not include any Objective-C types in `shell_test_platform_view_metal.h`, since that file was included into `shell_test_platform_view.cc`, which is pure C++. To work around this, we had encapsulated Objective-C Metal types in a `DarwinContextMetal` struct hidden in the implementation file with a pointer to it forward-declared in the header.
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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