Move Node::isKeyboardFocusable() to Element.
<http://webkit.org/b/116761>

Reviewed by Antti Koivisto.

Only Elements can be keyboard-focusable. Move the base isKeyboardFocusable() implementation to Element
and sprinkle OVERRIDE on subclass overrides.
FocusController gets some temporary type checks that will go away as that code moves to dealing in
Elements instead of Node.

* dom/Node.h:
* dom/Node.cpp:
* dom/Element.h:
* dom/Element.cpp:
(WebCore::Element::isKeyboardFocusable):

    Move isKeyboardFocusable() from Node to Element.

* page/FocusController.cpp:
(WebCore::isNonFocusableShadowHost):
(WebCore::isFocusableShadowHost):
(WebCore::shouldVisit):
(WebCore::FocusController::advanceFocusInDocumentOrder):

    Check that the inspected Node is an Element before querying isKeyboardFocusable().

* html/HTMLAnchorElement.h:
* html/HTMLAreaElement.h:
* html/HTMLFormControlElement.h:
* html/HTMLFrameOwnerElement.h:
* html/HTMLInputElement.h:
* html/HTMLPlugInElement.h:
* html/HTMLSelectElement.h:
* html/HTMLTextAreaElement.h:
* svg/SVGAElement.h:
* svg/SVGStyledElement.h:

    Sprinkle OVERRIDE.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150687 268f45cc-cd09-0410-ab3c-d52691b4dbfc