Move Shadow DOM renderers into rendering/

There are two renderers in html/shadow/
RenderSliderContainer and RenderSliderThumb.
They moved into rendering/ so that they are tracked like any other
renderers.

BUG=346445

Review URL: https://codereview.chromium.org/403523003

git-svn-id: svn://svn.chromium.org/blink/trunk@178456 bbb929c8-8fbe-4397-9dbb-9b2b20218538