Disable scrollbars on the root scroller when using visual viewport scrollbars.

third_party/WebKit/Source/platform/scroll/ScrollableArea.h

Index: third_party/WebKit/Source/platform/scroll/ScrollableArea.h
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollableArea.h b/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
index 2c20b6ea2cec866b90a4dd9a595f2243a7a740cb..4c7971f2b4646312c0a854f265ececeb51587a9b 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
@@ -122,6 +122,10 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin {
   virtual void didAddScrollbar(Scrollbar&, ScrollbarOrientation);
   virtual void willRemoveScrollbar(Scrollbar&, ScrollbarOrientation);
 
+  // Called when this ScrollableArea becomes or unbecomes the global root
+  // scroller.
+  virtual void didChangeGlobalRootScroller() {}
+
   virtual void contentsResized();
 
   bool hasOverlayScrollbars() const;