Prevent overlay scrollbars expand or hover together

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 bb3f810a4c2269d58267730ea97694916de2a899..a43bd48185ff7ee73909dd562a8b4a69e93b9819 100644
--- a/cc/input/scrollbar_animation_controller.h
+++ b/cc/input/scrollbar_animation_controller.h
@@ -37,7 +37,7 @@ class CC_EXPORT ScrollbarAnimationController {
  public:
   virtual ~ScrollbarAnimationController();
 
-  bool Animate(base::TimeTicks now);
+  virtual bool Animate(base::TimeTicks now);
 
   virtual void DidScrollBegin();
   virtual void DidScrollUpdate(bool on_resize);
@@ -45,7 +45,7 @@ class CC_EXPORT ScrollbarAnimationController {
   virtual void DidMouseDown() {}
   virtual void DidMouseUp() {}
   virtual void DidMouseLeave() {}
-  virtual void DidMouseMoveNear(float distance) {}
+  virtual void DidMouseMoveNear(ScrollbarOrientation, float) {}
   virtual bool ScrollbarsHidden() const;
 
  protected:
@@ -67,6 +67,8 @@ class CC_EXPORT ScrollbarAnimationController {
 
   int scroll_layer_id() const { return scroll_layer_id_; }
 
+  bool animating_fade() const { return is_animating_; }
+
  private:
   // Returns how far through the animation we are as a progress value from
   // 0 to 1.