Add onAsLuminance to SkLocalMatrixShader.

Change-Id: I3bec4b5a0d0ebe45a4684f509dab2fdfd5915926
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/854438
Commit-Queue: Nathan Sanchez <nathanasanchez@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
2 files changed