Move HTMLLabelElement-related complexity out of Element

Move HTMLLabelElement-related complexity out of Element and into the
HTMLLabelElement class.

R=tkent@chromium.org

Review URL: https://codereview.chromium.org/404833003

git-svn-id: svn://svn.chromium.org/blink/trunk@178607 bbb929c8-8fbe-4397-9dbb-9b2b20218538