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 9429d0733ceee41eca0c9abf7a3989e86103c3f2..fd9cf7b116e9a607a6cab6d64cd2cdd6a41014dd 100644
--- a/Source/core/rendering/RenderObject.h
+++ b/Source/core/rendering/RenderObject.h
@@ -67,6 +67,7 @@ class RenderNamedFlowThread;
 class RenderSVGResourceContainer;
 class RenderTable;
 class RenderTheme;
+class RenderView;
 class TransformState;
 
 struct PaintInfo;
@@ -589,6 +590,7 @@ public:
     virtual void updateDragState(bool dragOn);
 
     RenderView* view() const { return document()->renderView(); };
+    FrameView* frameView() const;
 
     // Returns true if this renderer is rooted, and optionally returns the hosting view (the root of the hierarchy).
     bool isRooted(RenderView** = 0) const;
@@ -674,6 +676,9 @@ 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() { }
@@ -1074,6 +1079,7 @@ private:
 
 #ifndef NDEBUG
     void checkBlockPositionedObjectsNeedLayout();
+    void checkNotInPartialLayout();
 #endif
 
     RefPtr<RenderStyle> m_style;
@@ -1244,6 +1250,9 @@ 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);
@@ -1257,6 +1266,9 @@ inline void RenderObject::setNeedsLayout(MarkingBehavior markParents, SubtreeLay
 
 inline void RenderObject::clearNeedsLayout()
 {
+#ifndef NDEBUG
+    checkNotInPartialLayout();
+#endif
     setSelfNeedsLayout(false);
     setEverHadLayout(true);
     setPosChildNeedsLayout(false);