aura: couple of tooltip UX changes:

ash/tooltips/tooltip_controller.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 ASH_TOOLTIPS_TOOLTIP_CONTROLLER_H_
 #define ASH_TOOLTIPS_TOOLTIP_CONTROLLER_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "base/string16.h"
 #include "base/timer.h"
@@ skipping 55 matching lines @@
   // |max_width| to the width (in pixels) of the clipped text and |line_count|
   // to the number of lines of text in the tooltip. |x| and |y| give the
   // location of the tooltip in screen coordinates.
   static void TrimTooltipToFit(string16* text,
                                int* max_width,
                                int* line_count,
                                int x,
                                int y);
 
   void TooltipTimerFired();
+  void TooltipShownTimerFired();
 
   // Updates the tooltip if required (if there is any change in the tooltip
   // text or the aura::Window.
   void UpdateIfRequired();
 
   // Only used in tests.
   bool IsTooltipVisible();
 
   bool IsDragDropInProgress();
 
   aura::client::DragDropClient* drag_drop_client_;
 
   aura::Window* tooltip_window_;
   string16 tooltip_text_;
 
   // These fields are for tracking state when the user presses a mouse button.
   aura::Window* tooltip_window_at_mouse_press_;
   string16 tooltip_text_at_mouse_press_;
   bool mouse_pressed_;
 
   scoped_ptr<Tooltip> tooltip_;
 
   base::RepeatingTimer<TooltipController> tooltip_timer_;
 
+  // Timer to timeout the life of an on-screen tooltip. We hide the tooltip when
+  // this timer fires.
+  base::OneShotTimer<TooltipController> tooltip_shown_timer_;
+
   gfx::Point curr_mouse_loc_;
 
   bool tooltips_enabled_;
 
   DISALLOW_COPY_AND_ASSIGN(TooltipController);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_TOOLTIPS_TOOLTIP_CONTROLLER_H_