Add effect caching to distance field text.

This also is a step towards unifying GrDistanceFieldTextureEffect and GrDistanceFieldLCDTextureEffect.

Committed: https://skia.googlesource.com/skia/+/137bac067306c5446bc4f9797bedc3bbaf302822

R=robertphillips@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/424103002
4 files changed