Move SubRunContainer to its own .h and .cpp

Move SubRunContainer, and all the associated classes used by it.

Change-Id: Ib61787f1e8442f0f8cc1354fbc04a867e3b6c670
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/545776
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
6 files changed