SkOnce for SkXfermode::Create(Mode)

This removes the mutex from the fast path when we've already cached it.

BUG=skia:
R=reed@google.com, bungeman@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/215533002

git-svn-id: http://skia.googlecode.com/svn/trunk@13986 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed