SubRunContainer: Cleanup includes and comment

Move comment closer to the implementation.

Change-Id: I5c5406b085e5a8b9ec4514360aed8e5d3743b90d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/589397
Auto-Submit: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed