aura: couple of tooltip UX changes:

ash/tooltips/tooltip_controller.h

Index: ash/tooltips/tooltip_controller.h
diff --git a/ash/tooltips/tooltip_controller.h b/ash/tooltips/tooltip_controller.h
index 6c36bcb6786844c616eb12d619110f6e30f5d8e1..c20cf88fbddc460c2f82706537a28b62ba07c671 100644
--- a/ash/tooltips/tooltip_controller.h
+++ b/ash/tooltips/tooltip_controller.h
@@ -73,6 +73,7 @@ class ASH_EXPORT TooltipController : public aura::client::TooltipClient,
                                int y);
 
   void TooltipTimerFired();
+  void TooltipShownTimerFired();
 
   // Updates the tooltip if required (if there is any change in the tooltip
   // text or the aura::Window.
@@ -97,6 +98,10 @@ class ASH_EXPORT TooltipController : public aura::client::TooltipClient,
 
   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_;