Add Traversal<>::firstAncestorOrSelf() overload taking a const reference in argument

Add Traversal<>::firstAncestorOrSelf() overload taking a const reference in
argument, and returning a const pointer. Make use of this new function in
Element::rendererIsFocusable().

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

Review URL: https://codereview.chromium.org/397303005

git-svn-id: svn://svn.chromium.org/blink/trunk@178504 bbb929c8-8fbe-4397-9dbb-9b2b20218538