clean up (partially) colortable api

Needs this to land: https://codereview.chromium.org/2789853002/

Bug: skia:
Change-Id: I38d916a546b7fa64d000d973e695ddda24a589e7
Reviewed-on: https://skia-review.googlesource.com/10600
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
37 files changed