solidify the type of a SubRun owner

Change-Id: Icd20e6d7d5fd25d1589f523c6a3cc6ef86adfeb6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/383756
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed