Hide use of TargetConditionals.h behind platform def
Hide use of TargetConditionals.h behind platform def

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 <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia