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

third_party/WebKit/Source/core/frame/FrameView.h

Index: third_party/WebKit/Source/core/frame/FrameView.h
diff --git a/third_party/WebKit/Source/core/frame/FrameView.h b/third_party/WebKit/Source/core/frame/FrameView.h
index e2abec05ec6e55d7ea49b5d188afd0bad1235319..fd1895912cb8ebb101196c9f422584c97c612943 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.h
+++ b/third_party/WebKit/Source/core/frame/FrameView.h
@@ -769,11 +769,10 @@ class CORE_EXPORT FrameView final
   void scrollContentsIfNeeded();
 
   enum ComputeScrollbarExistenceOption { FirstPass, Incremental };
-  void computeScrollbarExistence(
-      bool& newHasHorizontalScrollbar,
-      bool& newHasVerticalScrollbar,
-      const IntSize& docSize,
-      ComputeScrollbarExistenceOption = FirstPass) const;
+  void computeScrollbarExistence(bool& newHasHorizontalScrollbar,
+                                 bool& newHasVerticalScrollbar,
+                                 const IntSize& docSize,
+                                 ComputeScrollbarExistenceOption = FirstPass);
   void updateScrollbarGeometry();
 
   // Called to update the scrollbars to accurately reflect the state of the
@@ -868,6 +867,8 @@ class CORE_EXPORT FrameView final
   IntPoint convertToContainingWidget(const IntPoint&) const override;
   IntPoint convertFromContainingWidget(const IntPoint&) const override;
 
+  void didChangeGlobalRootScroller() override;
+
   void updateWidgetGeometriesIfNeeded();
 
   bool wasViewportResized();
@@ -915,7 +916,7 @@ class CORE_EXPORT FrameView final
   bool adjustScrollbarExistence(ComputeScrollbarExistenceOption = FirstPass);
   void adjustScrollbarOpacity();
   void adjustScrollOffsetFromUpdateScrollbars();
-  bool visualViewportSuppliesScrollbars() const;
+  bool visualViewportSuppliesScrollbars();

[skobes, 2016/11/15 20:18:43]

Why can't this be const anymore?

[bokan, 2016/11/15 22:04:10]

Because of the check: root scroller == layoutViewportScrollableArea(),
layoutViewportScrollableArea can't be const because if RLS is disabled it
returns `this`. 

The solution would be to add a const overload of layoutViewportScrollableArea()
but we've decided long ago that the ship has sailed on const-correctness.

 
   bool isFrameViewScrollbar(const Widget* child) const {
     return horizontalScrollbar() == child || verticalScrollbar() == child;