Reviewed by Oliver Hunt.

        Filled out some features in DoublyLinkedList
        https://bugs.webkit.org/show_bug.cgi?id=61549

        * heap/MarkedBlock.h: Use DoublyLinkedListNode to simplify our class
        definition.

        * wtf/DoublyLinkedList.h:
        (WTF::::DoublyLinkedListNode):
        (WTF::::setPrev):
        (WTF::::setNext):
        (WTF::::prev):
        (WTF::::next): Added a helper base class for being a node in a list.
        The odd idioms here allow subclasses to choose their own data member
        layout, which is sometimes important for performance.

        (WTF::::DoublyLinkedList):
        (WTF::::isEmpty):
        (WTF::::size):
        (WTF::::clear):
        (WTF::::head):
        (WTF::::append):
        (WTF::::remove):
        (WTF::::removeHead): Change "Node" to "T" to be more idiomatic, and added
        clear(), removeHead(), and size() functions.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@87408 268f45cc-cd09-0410-ab3c-d52691b4dbfc