Disable overlay scrollbars in Blink when hidden by the compositor.

cc/input/scrollbar_animation_controller.h

Index: cc/input/scrollbar_animation_controller.h
diff --git a/cc/input/scrollbar_animation_controller.h b/cc/input/scrollbar_animation_controller.h
index 7bb421f4946bf9d18251f64fbf31c61cf084a050..bb3f810a4c2269d58267730ea97694916de2a899 100644
--- a/cc/input/scrollbar_animation_controller.h
+++ b/cc/input/scrollbar_animation_controller.h
@@ -23,6 +23,7 @@ class CC_EXPORT ScrollbarAnimationControllerClient {
                                                  base::TimeDelta delay) = 0;
   virtual void SetNeedsRedrawForScrollbarAnimation() = 0;
   virtual void SetNeedsAnimateForScrollbarAnimation() = 0;
+  virtual void DidChangeScrollbarVisibility() = 0;
   virtual ScrollbarSet ScrollbarsFor(int scroll_layer_id) const = 0;
 
  protected:
@@ -45,6 +46,7 @@ class CC_EXPORT ScrollbarAnimationController {
   virtual void DidMouseUp() {}
   virtual void DidMouseLeave() {}
   virtual void DidMouseMoveNear(float distance) {}
+  virtual bool ScrollbarsHidden() const;
 
  protected:
   ScrollbarAnimationController(int scroll_layer_id,
@@ -63,6 +65,8 @@ class CC_EXPORT ScrollbarAnimationController {
 
   void PostDelayedAnimationTask(bool on_resize);
 
+  int scroll_layer_id() const { return scroll_layer_id_; }
+
  private:
   // Returns how far through the animation we are as a progress value from
   // 0 to 1.