AtomicNumber should make dealing with atomic counters easier. This is not the
case with size_t, as we cannot properly use the Increment() method for negative
numbers.

With this CL we can use AtomicNumber<size_t> and have proper decrements.

R=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2215693002
Cr-Commit-Position: refs/heads/master@{#38407}