cornerStart/cornerRect shouldn't require a RenderLayer

Those functions requires a RenderLayer where they actually have no
need for it: they only need the scrollbars and the RenderStyle.

Review URL: https://chromiumcodereview.appspot.com/19220005

git-svn-id: svn://svn.chromium.org/blink/trunk@154424 bbb929c8-8fbe-4397-9dbb-9b2b20218538