SkOnce: let f be any functor, update comments

BUG=
R=bungeman@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/104433005

git-svn-id: http://skia.googlecode.com/svn/trunk@12518 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed