MainFrame scrollbars should work with RFV instead of FV

third_party/WebKit/Source/core/paint/ScrollbarManager.cpp

Index: third_party/WebKit/Source/core/paint/ScrollbarManager.cpp
diff --git a/third_party/WebKit/Source/core/paint/ScrollbarManager.cpp b/third_party/WebKit/Source/core/paint/ScrollbarManager.cpp
index 48c01732a52c63857b77cace90c02d835eed21cc..a26706502f64eec505b8f64305c043f1003a06dd 100644
--- a/third_party/WebKit/Source/core/paint/ScrollbarManager.cpp
+++ b/third_party/WebKit/Source/core/paint/ScrollbarManager.cpp
@@ -17,6 +17,15 @@ DEFINE_TRACE(ScrollbarManager) {
   visitor->trace(m_vBar);
 }
 
+void ScrollbarManager::setScroller(ScrollableArea* scroller) {
+  DCHECK(m_scrollableArea != scroller);
+  DCHECK(scroller);
+  DCHECK(scroller->isRootFrameViewport() || scroller->isFrameView() ||
+         scroller->isPaintLayerScrollableArea());
+  dispose();
+  m_scrollableArea = scroller;
+}
+
 void ScrollbarManager::dispose() {
   m_hBarIsAttached = m_vBarIsAttached = 0;
   destroyScrollbar(HorizontalScrollbar);