http://bugs.webkit.org/show_bug.cgi?id=15148
        Bug 15148: Poor performance on crazy DOM raytracer
        
        Switch data structures to avoid quadratic behavior.

        * rendering/RenderBlock.cpp:
        (WebCore::RenderBlock::layoutPositionedObjects): Update for ListHashSet
        (WebCore::RenderBlock::markPositionedObjectsForLayout): Update for ListHashSet
        (WebCore::RenderBlock::insertPositionedObject): Use ListHashSet to avoid expensive uniqueing
        (WebCore::RenderBlock::removePositionedObject): Update for ListHashSet
        (WebCore::RenderBlock::removePositionedObjects): Update for ListHashSet
        (WebCore::RenderBlock::lowestPosition): Update for ListHashSet
        (WebCore::RenderBlock::rightmostPosition): Update for ListHashSet
        (WebCore::RenderBlock::leftmostPosition): Update for ListHashSet
        (WebCore::RenderBlock::rightBottom): Update for ListHashSet
        * rendering/RenderBlock.h: Change m_positionedObjects from DeprecatedPtrList to ListHashSet



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@25464 268f45cc-cd09-0410-ab3c-d52691b4dbfc