Add a few extra atomic operations to wtf/Atomics.h.

R=cevans@chromium.org, erik.corry@gmail.com, vegorov@chromium.org
BUG=

Review URL: https://codereview.chromium.org/104003007

git-svn-id: svn://svn.chromium.org/blink/trunk@164170 bbb929c8-8fbe-4397-9dbb-9b2b20218538