Move luminance calculation to GrTextBlob::Make()

This allows a couple of routines to be removed, and simplifies
the calling chaing.

Change-Id: I88b9396be79e4e245bbea291f4e778d12416b836
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/259196
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
3 files changed