change SkColorTable to be immutable

BUG=
R=scroggo@google.com

Review URL: https://codereview.chromium.org/25353002

git-svn-id: http://skia.googlecode.com/svn/trunk/include@11676 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed