Refactoring: Move Node::isFocusable to Element.

'focus' is a concept of elements.

BUG=258763

Review URL: https://chromiumcodereview.appspot.com/21856013

git-svn-id: svn://svn.chromium.org/blink/trunk@155509 bbb929c8-8fbe-4397-9dbb-9b2b20218538