Aura Overlay Scrollbars appear when mouse hovers over scroller edge

cc/layers/painted_scrollbar_layer_impl.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_LAYERS_PAINTED_SCROLLBAR_LAYER_IMPL_H_
 #define CC_LAYERS_PAINTED_SCROLLBAR_LAYER_IMPL_H_
 
 #include "base/macros.h"
 #include "cc/base/cc_export.h"
 #include "cc/input/scrollbar.h"
@@ skipping 37 matching lines @@
   }
 
   void set_thumb_opacity(float opacity) { thumb_opacity_ = opacity; }
 
   void set_internal_contents_scale_and_bounds(float content_scale,
                                               const gfx::Size& content_bounds) {
     internal_contents_scale_ = content_scale;
     internal_content_bounds_ = content_bounds;
   }
 
+  int ThumbThickness() const override;
+
  protected:
   PaintedScrollbarLayerImpl(LayerTreeImpl* tree_impl,
                             int id,
                             ScrollbarOrientation orientation,
                             bool is_left_side_vertical_scrollbar,
                             bool is_overlay);
 
   // ScrollbarLayerImplBase implementation.
-  int ThumbThickness() const override;
   int ThumbLength() const override;
   float TrackLength() const override;
   int TrackStart() const override;
   bool IsThumbResizable() const override;
 
  private:
   const char* LayerTypeAsString() const override;
 
   UIResourceId track_ui_resource_id_;
   UIResourceId thumb_ui_resource_id_;
 
   float thumb_opacity_;
 
   float internal_contents_scale_;
   gfx::Size internal_content_bounds_;
 
   int thumb_thickness_;
   int thumb_length_;
   int track_start_;
   int track_length_;
 
   DISALLOW_COPY_AND_ASSIGN(PaintedScrollbarLayerImpl);
 };
 
 }  // namespace cc
 #endif  // CC_LAYERS_PAINTED_SCROLLBAR_LAYER_IMPL_H_