Move luminace calculation to the SkScalerContext ctor.

N.B. The cananonical color calculation is still there to
avoid Chrome layout changes.

Change-Id: Ie7a48a48be7ab0a6cc90b2469e48d85990cf3c1d
Reviewed-on: https://skia-review.googlesource.com/c/187460
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed