Disable overlay scrollbars in Blink when hidden by the compositor.

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

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
index 38a974378f6b2f6a371837748ee07b4318ceca84..e18d697520cddd4373d515b28a61bb925bed1d5e 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
@@ -339,6 +339,8 @@ class CORE_EXPORT PaintLayerScrollableArea final
   void updateAfterLayout();
   void clampScrollOffsetsAfterLayout();
 
+  void didChangeScrollbarsHidden() override;
+
   void updateAfterStyleChange(const ComputedStyle*);
   void updateAfterOverflowRecalc();
 
@@ -493,6 +495,8 @@ class CORE_EXPORT PaintLayerScrollableArea final
  private:
   explicit PaintLayerScrollableArea(PaintLayer&);
 
+  void updateScrollbarsEnabledState();
+
   bool hasHorizontalOverflow() const;
   bool hasVerticalOverflow() const;
   bool hasScrollableHorizontalOverflow() const;