Detach PaintLayerScrollableArea from RootFrameViewport when disposed.

third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h

Index: third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h
diff --git a/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h b/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h
index b6bd17714142d1441bfac04689cc94c65831eca1..a293942104a35b6a0819ffe9ea77b1457220a747 100644
--- a/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h
+++ b/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h
@@ -14,6 +14,7 @@ class Document;
 class Element;
 class GraphicsLayer;
 class PaintLayer;
+class PaintLayerScrollableArea;
 class ScrollableArea;
 class ScrollStateCallback;
 
@@ -70,6 +71,10 @@ public:
     // replaced by the default root scroller.
     void didUpdateLayout();
 
+    // PaintLayerScrollableAreas need to notify this class when they're being
+    // disposed so that we can remove them as the root scroller.
+    void didDisposePaintLayerScrollableArea(PaintLayerScrollableArea&);
+
     // Returns the PaintLayer associated with the currently effective root
     // scroller.
     PaintLayer* rootScrollerPaintLayer() const;