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: [email protected], [email protected], [email protected] Author: [email protected] Review URL: https://codereview.chromium.org/304593003 git-svn-id: http://skia.googlecode.com/svn/trunk/include@14970 2bbb7eff-a529-9590-31e7-b0007b416f81