Reviewed by Adam Barth.

        Add more position constructors
        positionBeforeNode, positionAfterNode
        firstPositionInNode, lastPositionInNode
        https://bugs.webkit.org/show_bug.cgi?id=25494

        I also added a lastOffsetInNode and deployed it to a couple places.

        There are no callers to these new constructors yet, but those
        will be coming in future patches.

        * dom/Position.cpp:
        (WebCore::Position::computeOffsetInContainerNode):
        * dom/Position.h:
        (WebCore::positionBeforeNode):
        (WebCore::positionAfterNode):
        (WebCore::lastOffsetInNode):
        (WebCore::firstPositionInNode):
        (WebCore::lastPositionInNode):
        * editing/ApplyStyleCommand.cpp:
        * editing/TextIterator.cpp:
        (WebCore::SimplifiedBackwardsTextIterator::SimplifiedBackwardsTextIterator):
        * editing/htmlediting.cpp:
        (WebCore::lastOffsetForEditing):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@48235 268f45cc-cd09-0410-ab3c-d52691b4dbfc