Move RenderBox's push/pop clipping to an RAII object

My mind was boggling that we were manually ensuring these calls are
symmetrical, so I fixed the glitch.

R=pdr@chromium.org,jchaffraix@chromium.org,dsinclair@chromium.org

Review URL: https://codereview.chromium.org/663273002

git-svn-id: svn://svn.chromium.org/blink/trunk@184025 bbb929c8-8fbe-4397-9dbb-9b2b20218538