Avoid computing style twice when element has no existing style
https://bugs.webkit.org/show_bug.cgi?id=115042

Reviewed by Andreas Kling.
        
We currently compute element style twice if we end up having to reattach, first in recalcStyle and then
again during attach(). We can easily avoid this in common case where the element has no existing style
(usually because it is display:none). 

* dom/Element.cpp:
(WebCore::Element::recalcStyle):
        
    Don't compute the style and diff if there is no existing style. There is nothing to diff against.
    Instead just call reattach() directly, we ended up doing that anyway with the existing code.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148970 268f45cc-cd09-0410-ab3c-d52691b4dbfc