Make the lifetime of wallpaper manager more clear.

components/wallpaper/wallpaper_manager_base.h

 // Copyright 2014 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 COMPONENTS_WALLPAPER_WALLPAPER_MANAGER_BASE_H_
 #define COMPONENTS_WALLPAPER_WALLPAPER_MANAGER_BASE_H_
 
 #include <deque>
 #include <map>
 #include <string>
@@ skipping 221 matching lines @@
                                                const std::string& file);
 
   WallpaperManagerBase();
   ~WallpaperManagerBase() override;
 
   // Returns the appropriate wallpaper resolution for all root windows.
   virtual WallpaperResolution GetAppropriateResolution() = 0;
 
   virtual void SetCommandLineForTesting(base::CommandLine* command_line);
 
-  // Indicates imminent shutdown, allowing the WallpaperManager to remove any
-  // observers it has registered.
-  virtual void Shutdown() = 0;
-
   // Adds PowerManagerClient, TimeZoneSettings and CrosSettings observers.
   virtual void AddObservers() = 0;
 
   // Loads wallpaper asynchronously if the current wallpaper is not the
   // wallpaper of logged in user.
   virtual void EnsureLoggedInUserWallpaperLoaded();
 
   // Gets wallpaper information of logged in user.
   virtual bool GetLoggedInUserWallpaperInfo(WallpaperInfo* info);
 
@@ skipping 364 matching lines @@
 
   base::WeakPtrFactory<WallpaperManagerBase> weak_factory_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(WallpaperManagerBase);
 };
 
 }  // namespace chromeos
 
 #endif  // COMPONENTS_WALLPAPER_WALLPAPER_MANAGER_BASE_H_