remove sk_atomic_fetch_add

Change-Id: Ia400db2b73a8061668e62f8e961538a060b216a1
Reviewed-on: https://skia-review.googlesource.com/c/174282
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed