Move SubRun from .h to .cpp

Change-Id: I168df05e7cd77987d6bc501ceebf1b8aaa255e73
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/259170
Commit-Queue: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed