Fix several corner case issues of scrollbar paint invalidation

third_party/WebKit/Source/core/layout/LayoutObject.h

Index: third_party/WebKit/Source/core/layout/LayoutObject.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutObject.h b/third_party/WebKit/Source/core/layout/LayoutObject.h
index 452d1633e847ccc7eaea35cea054db101f70c4d0..68e14b8cf254a24ba456c5ac0ee72b846425fb1e 100644
--- a/third_party/WebKit/Source/core/layout/LayoutObject.h
+++ b/third_party/WebKit/Source/core/layout/LayoutObject.h
@@ -1233,7 +1233,9 @@ public:
     // this object).
     LayoutRect previousPaintInvalidationRectIncludingCompositedScrolling(const LayoutBoxModelObject& paintInvalidationContainer) const;
     LayoutSize previousPaintInvalidationRectSize() const { return previousPaintInvalidationRect().size(); }
-    void setPreviousPaintInvalidationRect(const LayoutRect& rect) { m_previousPaintInvalidationRect = rect; }
+
+    // Called when the previous paint invalidation rect(s) is no longer valid.
+    virtual void clearPreviousPaintInvalidationRects();
 
     // Only adjusts if the paint invalidation container is not a composited scroller.
     void adjustPreviousPaintInvalidationForScrollIfNeeded(const DoubleSize& scrollDelta);
@@ -1431,6 +1433,8 @@ protected:
     // of this layoutObject within the current layer that should be used for each result.
     virtual void computeSelfHitTestRects(Vector<LayoutRect>&, const LayoutPoint& layerOffset) const { }
 
+    void setPreviousPaintInvalidationRect(const LayoutRect& rect) { m_previousPaintInvalidationRect = rect; }
+
     virtual PaintInvalidationReason paintInvalidationReason(const LayoutBoxModelObject& paintInvalidationContainer,
         const LayoutRect& oldPaintInvalidationRect, const LayoutPoint& oldPositionFromPaintInvalidationBacking,
         const LayoutRect& newPaintInvalidationRect, const LayoutPoint& newPositionFromPaintInvalidationBacking) const;