Add a skia method to perform an atomic add.

Complements sk_atomic_inc for when you want to increase by more than one.

This time, use the correct atomic add function on Windows.

Reviewed at https://codereview.appspot.com/6399050/

Review URL: https://codereview.appspot.com/6407048

git-svn-id: http://skia.googlecode.com/svn/trunk@4623 2bbb7eff-a529-9590-31e7-b0007b416f81
7 files changed