[graphite] Add the combination system's BeginBlock usages

This is just the lowest level of the combination system. The higher
level iteration to accumulate the keys will be in a following CL.

Bug: skia:12701
Change-Id: I4015790a7bca49f11f8eb1a4fda32f235845c049
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/547819
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed