[graphite] Convert key helpers from being namespaces to classes

Bug: skia:12701
Change-Id: I17ff95cd7f7fc518158e951d24c109fc5d4d003d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/548417
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed