[graphite] Fill in PrecompileBlendShader's addToKey method

Bug: b/259547998
Change-Id: I4dabcc39c7077144f2728944031a0de200bc1af4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/615079
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed