Remove StyleResolverState::styledElement() since we can just check isHTMLElement() or isSVGElement()

StyledElement just means "HTML or SVG element" so we can just check that in the getter
and use element() in the StyleResolver. This removes one more bit of state from the
style resolver and simplifies the code.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/18139007

git-svn-id: svn://svn.chromium.org/blink/trunk@154096 bbb929c8-8fbe-4397-9dbb-9b2b20218538