add missing <atomic> include

TBR=brianosman@google.com, johnstiles@google.com

Change-Id: I03f771b39acbeba3d25da97d759ad2f9a8f6f5d3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/324123
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed