Add a skia method to perform an atomic add.

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

Review URL: https://codereview.appspot.com/6350106

git-svn-id: http://skia.googlecode.com/svn/trunk@4614 2bbb7eff-a529-9590-31e7-b0007b416f81
7 files changed