Reviewed by Daniel Bates.

        Spatial Navigation: code clean up (part II)
        https://bugs.webkit.org/show_bug.cgi?id=50666

        No new tests needed.

        * page/FocusController.cpp: Removed static declaration of updateFocusCandidateIfNeeded() from
        the top of FocusController.cpp, and added the 'static' keyword where the function is implemented;
        * page/SpatialNavigation.cpp: In FocusCandidate constructor, renamed 'n' to 'node', and added an assert to it;
        (WebCore::FocusCandidate::FocusCandidate):
        (WebCore::virtualRectForAreaElementAndDirection): Added an assert to 'node';
        * page/SpatialNavigation.h:I reordered the declaration of some methods in order to group related ones;
        Removed isScrollableContainerNode() function declaration since it is not used outside SpatialNavigation.cpp;
        And removed the declaration of isNodeDeepDescendantOfDocument() since it does not exist anymore.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@73885 268f45cc-cd09-0410-ab3c-d52691b4dbfc