add facilities for flattening different subrun types

Change-Id: Ie223b7ef7035df0c2e40b4b2dd026170a23dbd18
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/509557
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed