Add gamma/sRGB tags to SkImageInfo

requires this CL to land in chrome
https://codereview.chromium.org/517803002/

BUG=skia:
R=fmalita@google.com, fmalita@chromium.org, reed@chromium.org

Author: reed@google.com

Review URL: https://codereview.chromium.org/517123002
4 files changed