Create Tooltip object lazily so that it gets proper drop shadow

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 c20cf88fbddc460c2f82706537a28b62ba07c671..68ddd7f147aaa7782dc66342385354fef2df2d1c 100644
--- a/ash/tooltips/tooltip_controller.h
+++ b/ash/tooltips/tooltip_controller.h
@@ -84,6 +84,10 @@ class ASH_EXPORT TooltipController : public aura::client::TooltipClient,
 
   bool IsDragDropInProgress();
 
+  // This lazily creates the Tooltip instance so that the tooltip window will
+  // be initialized with appropriate drop shadows.
+  Tooltip* GetTooltip();
+
   aura::client::DragDropClient* drag_drop_client_;
 
   aura::Window* tooltip_window_;