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/include@14970 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed