share impl between makeNonTexture and makeRaster

Bug: skia:
Change-Id: I1042e25c28b60e99b6ced44b9971d51a3ac239ee
Reviewed-on: https://skia-review.googlesource.com/90201
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed