Don't check isElementNode() yourself inside Element::recalcChildStyle

Add Element::youngestShadowRoot() to call on Element* instead of using
Node::youngestShadowRoot() and checking isElementNode().

Review URL: https://codereview.chromium.org/59703006

git-svn-id: svn://svn.chromium.org/blink/trunk@161416 bbb929c8-8fbe-4397-9dbb-9b2b20218538