Fix selection gap painting in Slimming Paint

Selection gap painting is wonky. It happens in functions that
don't necessarily paint, and aren't in the painter classes
(sad panda). Fixing it also forces us to either make the
paint functions no longer const (can of worms), or it forces
the slimming paint code to operate on const RenderObjects.
The latter is preferable, but will cause problems if we start
clearing state bits during paint.

Review URL: https://codereview.chromium.org/684423004

git-svn-id: svn://svn.chromium.org/blink/trunk@184761 bbb929c8-8fbe-4397-9dbb-9b2b20218538