Disable overlay scrollbars in Blink when hidden by the compositor.

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 d921b2d2da5ffc99455b6c1fbf51ea7aba599a2b..8126029a7457f44ce1316525131df816d9003dff 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
+++ b/third_party/WebKit/Source/platform/scroll/ScrollableArea.h
@@ -252,6 +252,11 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin {
   virtual bool usesCompositedScrolling() const { return false; }
   virtual bool shouldScrollOnMainThread() const;
 
+  // Overlay scrollbars can "fade-out" when inactive.
+  virtual bool scrollbarsHidden() const;
+  virtual void setScrollbarsHidden(bool);
+  virtual void didChangeScrollbarsHidden(){};
+
   // Returns true if the GraphicsLayer tree needs to be rebuilt.
   virtual bool updateAfterCompositingChange() { return false; }
 
@@ -397,6 +402,7 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin {
   unsigned m_horizontalScrollbarNeedsPaintInvalidation : 1;
   unsigned m_verticalScrollbarNeedsPaintInvalidation : 1;
   unsigned m_scrollCornerNeedsPaintInvalidation : 1;
+  unsigned m_scrollbarsHidden : 1;
 
   // There are 6 possible combinations of writing mode and direction. Scroll
   // origin will be non-zero in the x or y axis if there is any reversed