MacViews: Reveal scrollbars when resting on the trackpad.

ui/views/controls/scrollbar/scroll_bar.h

 // Copyright (c) 2012 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 UI_VIEWS_CONTROLS_SCROLLBAR_SCROLL_BAR_H_
 #define UI_VIEWS_CONTROLS_SCROLLBAR_SCROLL_BAR_H_
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "ui/views/view.h"
@@ skipping 72 matching lines @@
 
   // Get the width or height of this scrollbar, for use in layout calculations.
   // For a vertical scrollbar, this is the width of the scrollbar, likewise it
   // is the height for a horizontal scrollbar.
   virtual int GetLayoutSize() const = 0;
 
   // Get the width or height for this scrollbar which overlaps with the content.
   // Default is 0.
   virtual int GetContentOverlapSize() const;
 
+  // Called when a ScrollEvent (in any, or no, direction) is seen by the parent
+  // ScrollView. E.g., this may reveal an overlay scrollbar to indicate
+  // possible scrolling directions to the user.
+  virtual void ObserveScrollEvent(const ui::ScrollEvent& event);
+
  protected:
   // Create new scrollbar, either horizontal or vertical. These are protected
   // since you need to be creating either a NativeScrollBar or a
   // ImageScrollBar.
   explicit ScrollBar(bool is_horiz);
 
  private:
   const bool is_horiz_;
 
   ScrollBarController* controller_;
 
   int max_pos_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollBar);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_SCROLLBAR_SCROLL_BAR_H_