Remove form control related virtual methods from Element

As the FIXME says, these are only used by form controls. That is, setChangedSinceLastFormControlChangeEvent
can be removed from Element as-is. The other two can also be removed but only after adding some cast code
to Document.setFocusedElement.

Based on http://trac.webkit.org/changeset/150711 by Andreas Kling.

Review URL: https://codereview.chromium.org/137833018

git-svn-id: svn://svn.chromium.org/blink/trunk@166216 bbb929c8-8fbe-4397-9dbb-9b2b20218538