Do not activate system tray bubble by default

ash/common/shelf/shelf_layout_manager.h

 // Copyright 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_COMMON_SHELF_SHELF_LAYOUT_MANAGER_H_
 #define ASH_COMMON_SHELF_SHELF_LAYOUT_MANAGER_H_
 
 #include <memory>
 
 #include "ash/ash_export.h"
@@ skipping 172 matching lines @@
   ShelfBackgroundType GetShelfBackgroundType() const;
 
   // Set the height of the ChromeVox panel, which takes away space from the
   // available work area from the top of the screen.
   void SetChromeVoxPanelHeight(int height);
 
  private:
   class UpdateShelfObserver;
   friend class PanelLayoutManagerTest;
   friend class ShelfLayoutManagerTest;
+  friend class WebNotificationTrayTest;
 
   struct TargetBounds {
     TargetBounds();
     ~TargetBounds();
 
     float opacity;
     float status_opacity;
     gfx::Rect shelf_bounds_in_root;
     gfx::Rect shelf_bounds_in_shelf;
     gfx::Rect status_bounds_in_shelf;
@@ skipping 168 matching lines @@
   // The flag to enforce invisible shelf (as in MD-experiemntal).
   // TODO(oshima): Remove this when MD immersive is launched.
   bool invisible_auto_hide_shelf_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(ShelfLayoutManager);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SHELF_SHELF_LAYOUT_MANAGER_H_