Add a safe cast for RenderSliderThumb

Rather than a dangerous static_cast use the
DEFINE_RENDER_OBJECT_TYPE_CASTS machinery.

Review URL: https://codereview.chromium.org/611983006

git-svn-id: svn://svn.chromium.org/blink/trunk@183223 bbb929c8-8fbe-4397-9dbb-9b2b20218538