Fix use of nested designators (#45869)

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 https://github.com/flutter/buildroot/blob/b9fbd310d6fa68f888738d5236ea2f6caa2d2bfc/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

Author: jiahaog

shell/platform/darwin/macos/framework/Source/FlutterRenderer.mm

@@ -62,16 +62,17 @@ - (instancetype)initWithFlutterEngine:(nonnull FlutterEngine*)flutterEngine {
 - (FlutterRendererConfig)createRendererConfig {
   FlutterRendererConfig config = {
       .type = FlutterRendererType::kMetal,
-      .metal.struct_size = sizeof(FlutterMetalRendererConfig),
-      .metal.device = (__bridge FlutterMetalDeviceHandle)_device,
-      .metal.present_command_queue = (__bridge FlutterMetalCommandQueueHandle)_commandQueue,
-      .metal.get_next_drawable_callback =
-          reinterpret_cast<FlutterMetalTextureCallback>(OnGetNextDrawable),
-      .metal.present_drawable_callback =
-          reinterpret_cast<FlutterMetalPresentCallback>(OnPresentDrawable),
-      .metal.external_texture_frame_callback =
-          reinterpret_cast<FlutterMetalTextureFrameCallback>(OnAcquireExternalTexture),
-  };
+      .metal = {
+          .struct_size = sizeof(FlutterMetalRendererConfig),
+          .device = (__bridge FlutterMetalDeviceHandle)_device,
+          .present_command_queue = (__bridge FlutterMetalCommandQueueHandle)_commandQueue,
+          .get_next_drawable_callback =
+              reinterpret_cast<FlutterMetalTextureCallback>(OnGetNextDrawable),
+          .present_drawable_callback =
+              reinterpret_cast<FlutterMetalPresentCallback>(OnPresentDrawable),
+          .external_texture_frame_callback =
+              reinterpret_cast<FlutterMetalTextureFrameCallback>(OnAcquireExternalTexture),
+      }};
   return config;
 }