Drop support for non-atomic non-mutex platforms.

(Clients can still provide these themselves if they really want.)

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

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/295373005

git-svn-id: http://skia.googlecode.com/svn/trunk@14894 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed