Add SkBarriers to ports. This completes a TODO we've had to move our memory-barrier code out of SkOnce. I also want to start using sk_acquire_load elsewhere. BUG=skia: R=bungeman@google.com, mtklein@google.com, reed@google.com Author: mtklein@chromium.org Review URL: https://codereview.chromium.org/304593003 git-svn-id: http://skia.googlecode.com/svn/trunk@14970 2bbb7eff-a529-9590-31e7-b0007b416f81