Use NodeTraversal instead of ElementTraversal in VisitedLinkState

Use NodeTraversal instead of ElementTraversal in VisitedLinkState to avoid
redundant Node::isElementNode() checks. Since isLink() is defined on Node, and
not on Element, we can simply iterate through all Nodes and then calls isLink()
on them (instead of isElementNode() && toElement(node).isLink()).

R=esprehn@chromium.org, adamk@chromium.org

Review URL: https://codereview.chromium.org/483383002

git-svn-id: svn://svn.chromium.org/blink/trunk@180677 bbb929c8-8fbe-4397-9dbb-9b2b20218538