Difficulty linking ash struct traits unit tests.

This came up while adding shelf_struct_traits_unittest.cc in Patch Set 3 here:
  https://codereview.chromium.org/2750463009/#ps40001

For some reason, including ":ash_public_cpp", in the new unit_tests target isn't sufficient to pull in the struct traits definitions; I get these ash_unittests linker errors from that CL:

[1/1] LINK ./ash_unittests
FAILED: ash_unittests 
...
../../mojo/public/cpp/bindings/lib/serialization.h:89: error: undefined reference to 'ash::mojom::internal::ShelfItem_Data::Validate(void const*, mojo::internal::ValidationContext*)'
gen/ash/public/interfaces/shelf.mojom-shared.h:1183: error: undefined reference to 'mojo::StructTraits<ash::mojom::ShelfItemDataView, ash::ShelfItem>::Read(ash::mojom::ShelfItemDataView, ash::ShelfItem*)'
gen/ash/public/interfaces/shelf.mojom-shared.h:1107: error: undefined reference to 'mojo::StructTraits<ash::mojom::ShelfItemDataView, ash::ShelfItem>::image(ash::ShelfItem const&)'
gen/ash/public/interfaces/shelf.mojom-shared.h:1112: error: undefined reference to 'mojo::StructTraits<ash::mojom::ShelfItemDataView, ash::ShelfItem>::app_id(ash::ShelfItem const&)'
gen/ash/public/interfaces/shelf.mojom-shared.h:1115: error: undefined reference to 'mojo::StructTraits<ash::mojom::ShelfItemDataView, ash::ShelfItem>::title(ash::ShelfItem const&)'
gen/ash/public/interfaces/shelf.mojom-shared.h:1135: error: undefined reference to 'mojo::StructTraits<ash::mojom::ShelfItemDataView, ash::ShelfItem>::type(ash::ShelfItem const&)'
gen/ash/public/interfaces/shelf.mojom-shared.h:1137: error: undefined reference to 'mojo::StructTraits<ash::mojom::ShelfItemDataView, ash::ShelfItem>::image(ash::ShelfItem const&)'
gen/ash/public/interfaces/shelf.mojom-shared.h:0: error: undefined reference to 'mojo::StructTraits<ash::mojom::ShelfItemDataView, ash::ShelfItem>::shelf_id(ash::ShelfItem const&)'
gen/ash/public/interfaces/shelf.mojom-shared.h:1148: error: undefined reference to 'mojo::StructTraits<ash::mojom::ShelfItemDataView, ash::ShelfItem>::status(ash::ShelfItem const&)'
gen/ash/public/interfaces/shelf.mojom-shared.h:1149: error: undefined reference to 'mojo::StructTraits<ash::mojom::ShelfItemDataView, ash::ShelfItem>::app_id(ash::ShelfItem const&)'

If I add a direct unit_tests dependency on "//ash/public/interfaces:interfaces_internal", that works (at least for my debug component build).
I'll see if that's a valid workaround, but Yuzhu points out that it might not work for non-component builds...

We found one potential idea that might point towards a better solution, by exporting ShelfItem_Data in the generated file out/Default/gen/ash/public/interfaces/shelf.mojom-shared-internal.h
<...>
#define ASH_PUBLIC_IMPLEMENTATION 1
#include "ash/public/cpp/ash_public_export.h"
<...>
class ASH_PUBLIC_EXPORT ShelfItem_Data