make make{SRGB,Linear}Gamma() const

Change-Id: If20a43a905ae65643d071978b345d9ff09c04cf1
Reviewed-on: https://skia-review.googlesource.com/19541
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed