Fix clipping in Slimming Paint

Previously, we weren't balancing our save/restore, and
weren't actually properly clipping

Review URL: https://codereview.chromium.org/668593003

git-svn-id: svn://svn.chromium.org/blink/trunk@184602 bbb929c8-8fbe-4397-9dbb-9b2b20218538