add helper static to SkColorSpaceXform

Bug: skia:
Change-Id: I62525b392dfbae3d7075cf7f14e30780bad41279
Reviewed-on: https://skia-review.googlesource.com/17485
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed