Reviewed by Eric Seidel.

        Reorganizing helper functions in htmlediting.h to group them by type of object they return/operate upon.
        https://bugs.webkit.org/show_bug.cgi?id=29319

        * editing/IndentOutdentCommand.cpp:
        Removed implementation of isAtUnsplittableElement.

        * editing/IndentOutdentCommand.h:
        Removed member function isAtUnsplittableElement, because it was not class specific and belonged logically among the helper functions.

        * editing/htmlediting.cpp:
        (WebCore::isAtUnsplittableElement): added. The code was taken from IndentOutdentCommand.

        * editing/htmlediting.h:
        Reordered functions to group them by type of object returned or operate upon.
        Added comments.
        Added isAtUnsplittableElement declaration.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@48448 268f45cc-cd09-0410-ab3c-d52691b4dbfc