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 78ff7e05d7fda3f1c1793cce46584975b0b49159..42a84570f60de580d80af3615cfecafb5e83e0b1 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;
@@ -674,6 +675,9 @@ 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() { }
@@ -1073,6 +1077,7 @@ private:
 
 #ifndef NDEBUG
     void checkBlockPositionedObjectsNeedLayout();
+    void checkNotInPartialLayout();
 #endif
 
     RefPtr<RenderStyle> m_style;
@@ -1243,6 +1248,9 @@ inline bool RenderObject::isBeforeOrAfterContent() const
 
 inline void RenderObject::setNeedsLayout(MarkingBehavior markParents, SubtreeLayoutScope* layouter)
 {
+#ifndef NDEBUG
+    checkNotInPartialLayout();
+#endif
     ASSERT(!isSetNeedsLayoutForbidden());
     bool alreadyNeededLayout = m_bitfields.needsLayout();
     m_bitfields.setNeedsLayout(true);
@@ -1256,6 +1264,9 @@ inline void RenderObject::setNeedsLayout(MarkingBehavior markParents, SubtreeLay
 
 inline void RenderObject::clearNeedsLayout()
 {
+#ifndef NDEBUG
+    checkNotInPartialLayout();
+#endif
     m_bitfields.setNeedsLayout(false);
     setEverHadLayout(true);
     setPosChildNeedsLayout(false);