It's possible to enable SK_METAL on non-Apple platforms which will lead
to compile issues. This will prevent that. Also includes a minor change
to keep the decl of GrMtlTextureInfo consistent with other uses.
Change-Id: I48ae89b53c3ea8b50ee78383c5a81fdd09e9e3e3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/388038
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
45564260d4