Encapsulate the checks for shouldRepaintOverflow into RenderObject.

Instead of explosing an IfNeeded method that returns a boolean on
RenderObject we should just add a method that does the work IfNeeded
like we do in the rest of blink.

Review URL: https://codereview.chromium.org/200763007

git-svn-id: svn://svn.chromium.org/blink/trunk@169300 bbb929c8-8fbe-4397-9dbb-9b2b20218538