Invalidate paint when layout an RenderSliderContainer object.

This is necessary because this class uses one-off code that does not go through the regular style/layout system.

BUG=392694

Review URL: https://codereview.chromium.org/418643003

git-svn-id: svn://svn.chromium.org/blink/trunk@178883 bbb929c8-8fbe-4397-9dbb-9b2b20218538