Disable overlay scrollbars in Blink when hidden by the compositor.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 303 matching lines @@
   // post-update work even if the scroll position didn't change.
   void setScrollPositionUnconditionally(const DoublePoint&,
                                         ScrollType = ProgrammaticScroll);
 
   // TODO(szager): Actually run these after all of layout is finished.
   // Currently, they run at the end of box()'es layout (or after all flexbox
   // layout has finished) but while document layout is still happening.
   void updateAfterLayout();
   void clampScrollOffsetsAfterLayout();
 
+  void didChangeScrollbarsHidden() override;
+
   void updateAfterStyleChange(const ComputedStyle*);
   void updateAfterOverflowRecalc();
 
   bool updateAfterCompositingChange() override;
 
   bool hasScrollbar() const {
     return hasHorizontalScrollbar() || hasVerticalScrollbar();
   }
   bool hasOverflowControls() const {
     return hasScrollbar() || scrollCorner() || resizer();
@@ skipping 134 matching lines @@
   void invalidateStickyConstraintsFor(PaintLayer*,
                                       bool needsCompositingUpdate = true);
 
   uint64_t id() const;
 
   DECLARE_VIRTUAL_TRACE();
 
  private:
   explicit PaintLayerScrollableArea(PaintLayer&);
 
+  void updateScrollbarsEnabledState();
+
   bool hasHorizontalOverflow() const;
   bool hasVerticalOverflow() const;
   bool hasScrollableHorizontalOverflow() const;
   bool hasScrollableVerticalOverflow() const;
   bool visualViewportSuppliesScrollbars() const;
 
   bool needsScrollbarReconstruction() const;
 
   void updateScrollOrigin();
   void updateScrollDimensions();
@@ skipping 89 matching lines @@
 
 DEFINE_TYPE_CASTS(PaintLayerScrollableArea,
                   ScrollableArea,
                   scrollableArea,
                   scrollableArea->isPaintLayerScrollableArea(),
                   scrollableArea.isPaintLayerScrollableArea());
 
 }  // namespace blink
 
 #endif  // LayerScrollableArea_h