Remove RenderObject::outlineStyle()

Now we push outline style to continuations
(after https://src.chromium.org/viewvc/blink?view=rev&revision=179756)
so RenderObject::outlineStyle() is no longer needed.

Review URL: https://codereview.chromium.org/479893003

git-svn-id: svn://svn.chromium.org/blink/trunk@180993 bbb929c8-8fbe-4397-9dbb-9b2b20218538