Add const to GrColorSpaceXform member arguments

BUG=skia:

Change-Id: I643c486066a0cd48e9c985ab5f3ee453c2a533c5
Reviewed-on: https://skia-review.googlesource.com/9644
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed