gn: move all skia_* declare_args to skia.gni

motivation:  put them all in one place.

Change-Id: Id40aff1f6ddc36b99b14567f110b5084da7e2bd4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/238899
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
2 files changed