Reviewed by Maciej Stachowiak.

        added InlineBox::isLeaf()
        firstLeafChild()/lastLeafChild() not virtual and not callable on InlineBox anymore.
        firstLeafChild()/lastLeafChild() will no longer return a node outside of the given subtree.   
        Removed firstLeafChildAfterBox()/lastLeafChildBeforeBox()
        Removed potentially quadratic behavior if all nodes before/after a given box are empty InlineFlowBoxes

        Currently, these methods are called on RootInlineBox objects only, so above changes should not have
        any observable effect (only the removal of the square performance behavior could apply, 
        but the conditions for that are probably of a rather theoretical nature).

        * rendering/InlineBox.cpp:
        (WebCore::InlineBox::nextLeafChild):
        (WebCore::InlineBox::prevLeafChild):
        * rendering/InlineBox.h:
        (WebCore::InlineBox::isLeaf):
        * rendering/InlineFlowBox.cpp:
        (WebCore::InlineFlowBox::firstLeafChild):
        (WebCore::InlineFlowBox::lastLeafChild):
        * rendering/InlineFlowBox.h:
        (WebCore::InlineFlowBox::firstChild):
        (WebCore::InlineFlowBox::lastChild):
        (WebCore::InlineFlowBox::isLeaf):
        * rendering/RootInlineBox.cpp:
        (WebCore::RootInlineBox::closestLeafChildForXPos):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@45702 268f45cc-cd09-0410-ab3c-d52691b4dbfc