More clean-up while fixing <rdar://problem/5112273> REGRESSION(TOT): 
        Reproducible crash loading an old version of amazon.com as a web archive
        
        Layout tests pass.
        
        Removed more superfluous document NULL checks. (Node::document() never 
        returns NULL, and it asserts as much.)
        
        Merged some duplicate editing code into a static inline function.
        
        * page/mac/WebCoreFrameBridge.mm:
        (updateRenderingForBindings): Removed superfluous static_cast.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@20725 268f45cc-cd09-0410-ab3c-d52691b4dbfc