Add containerNode(), computeOffsetInContainerNode(),
        computeNodeBeforePosition() and computeNodeAfterPosition().
        https://bugs.webkit.org/show_bug.cgi?id=24763

        These functions will replace uses of rangeCompliantPosition()
        in future patches.

        No changes in behavior, thus no tests.

        * dom/Position.cpp:
        (WebCore::Position::containerNode):
        (WebCore::Position::computeOffsetInContainerNode):
        (WebCore::Position::computeNodeBeforePosition):
        (WebCore::Position::computeNodeAfterPosition):
        (WebCore::Position::anchorType):
        * dom/Position.h:
        (WebCore::Position::):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@42263 268f45cc-cd09-0410-ab3c-d52691b4dbfc