Add infrastructure for partial layouts

Source/core/rendering/RenderObject.h

Index: Source/core/rendering/RenderObject.h
diff --git a/Source/core/rendering/RenderObject.h b/Source/core/rendering/RenderObject.h
index 16264201d1ada3c0f0700e92de4cf91429c8f572..99933164213da6774c6fddb84e08360276617d08 100644
--- a/Source/core/rendering/RenderObject.h
+++ b/Source/core/rendering/RenderObject.h
@@ -64,6 +64,8 @@ class RenderLayerModelObject;
 class RenderNamedFlowThread;
 class RenderSVGResourceContainer;
 class RenderTable;
+class RenderTheme;
+class RenderView;
 class TransformState;
 
 struct PaintInfo;
@@ -584,6 +586,7 @@ public:
     virtual void updateDragState(bool dragOn);
 
     RenderView* view() const { return document().renderView(); };
+    FrameView* frameView() const { return document().view(); };
 
     // Returns true if this renderer is rooted, and optionally returns the hosting view (the root of the hierarchy).
     bool isRooted(RenderView** = 0) const;
@@ -670,6 +673,9 @@ public:
     void forceLayout();
     void forceChildLayout();
 
+    // True if we can abort layout, leaving a partially laid out tree.
+    virtual bool supportsPartialLayout() const { return false; }
+
     // used for element state updates that cannot be fixed with a
     // repaint and do not need a relayout
     virtual void updateFromElement() { }
@@ -1070,6 +1076,7 @@ private:
 
 #ifndef NDEBUG
     void checkBlockPositionedObjectsNeedLayout();
+    void checkNotInPartialLayout();
 #endif
 
     RefPtr<RenderStyle> m_style;
@@ -1240,6 +1247,9 @@ inline bool RenderObject::isBeforeOrAfterContent() const
 
 inline void RenderObject::setNeedsLayout(MarkingBehavior markParents, SubtreeLayoutScope* layouter)
 {
+#ifndef NDEBUG
+    checkNotInPartialLayout();
+#endif
     ASSERT(!isSetNeedsLayoutForbidden());
     bool alreadyNeededLayout = m_bitfields.selfNeedsLayout();
     setSelfNeedsLayout(true);
@@ -1253,6 +1263,9 @@ inline void RenderObject::setNeedsLayout(MarkingBehavior markParents, SubtreeLay
 
 inline void RenderObject::clearNeedsLayout()
 {
+#ifndef NDEBUG
+    checkNotInPartialLayout();
+#endif
     setSelfNeedsLayout(false);
     setEverHadLayout(true);
     setPosChildNeedsLayout(false);