There was an internal global change which turned `-Wc99-designator`. This causes the following error:
```
error: nested designators are a C99 extension [-Werror,-Wc99-designator]
65 | .metal.struct_size = sizeof(FlutterMetalRendererConfig),
| ^~~~~~~~~~~~~~~~~~
```
This seems like a trivial (?) fix, so perhaps we can fix it properly and possibly remove b9fbd310d6/build/config/compiler/BUILD.gn (L667) after this PR lands.
*List which issues are fixed by this PR. You must list at least one issue.*
b/300573112
*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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