This is cleanup left over from a prior CL's review. We will want to
use the combination builder outside of graphite.
This CL is mainly just moving stuff around except for the addition
of the SkCombinationBuilder.buildCombinations method and CreateKey
more accessible outside of Graphite.
Bug: skia:12701
Change-Id: If2cae6fcff5670e488bc14473b7b1d2f9b1fecd7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/543196
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
