use std::atomic in SkRegionPriv

Bug: skia:7882
Change-Id: I333f3457fbcd9621d712c40844f4896bd85a9a1c
Reviewed-on: https://skia-review.googlesource.com/124266
Reviewed-by: Yuqian Li <liyuqian@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed