Overlay scrollbars expand only when mouse is near thumb

cc/layers/scrollbar_layer_impl_base.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_SCROLLBAR_LAYER_IMPL_BASE_H_
 #define CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_
 
 #include "base/macros.h"
 #include "cc/cc_export.h"
 #include "cc/input/scrollbar.h"
@@ skipping 28 matching lines @@
   bool is_left_side_vertical_scrollbar() {
     return is_left_side_vertical_scrollbar_;
   }
 
   bool CanScrollOrientation() const;
 
   void PushPropertiesTo(LayerImpl* layer) override;
   ScrollbarLayerImplBase* ToScrollbarLayer() override;
 
   // Thumb quad rect in layer space.
-  virtual gfx::Rect ComputeThumbQuadRect() const;
+  gfx::Rect ComputeThumbQuadRect() const;
+  gfx::Rect ComputeExpandedThumbQuadRect() const;
 
   float thumb_thickness_scale_factor() {
     return thumb_thickness_scale_factor_;
   }
   bool SetThumbThicknessScaleFactor(float thumb_thickness_scale_factor);
 
   virtual int ThumbThickness() const = 0;
 
   // TODO(crbug.com/702832): No need for this function once there is element id
   // on overlay scrollbar layers.
@@ skipping 30 matching lines @@
   float vertical_adjust_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarLayerImplBase);
 };
 
 typedef std::set<ScrollbarLayerImplBase*> ScrollbarSet;
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_SCROLLBAR_LAYER_IMPL_BASE_H_