Make the lifetime of wallpaper manager more clear.

chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.h

Index: chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.h
diff --git a/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.h b/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.h
index 536d752dcfb088f21aec427677d048e5cf58592c..8b2b4d1c8e69e880e2c2fcb27fa94f1ab60280ad 100644
--- a/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.h
+++ b/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.h
@@ -37,18 +37,17 @@ class WallpaperManager :
  public:
   class PendingWallpaper;
 
-  WallpaperManager();
   ~WallpaperManager() override;
 
   // Get pointer to singleton WallpaperManager instance, create it if necessary.
   static WallpaperManager* Get();
 
-  // Returns the appropriate wallpaper resolution for all root windows.
-  WallpaperResolution GetAppropriateResolution() override;
-
   // Indicates imminent shutdown, allowing the WallpaperManager to remove any
   // observers it has registered.
-  void Shutdown() override;
+  static void Shutdown();
+
+  // Returns the appropriate wallpaper resolution for all root windows.
+  WallpaperResolution GetAppropriateResolution() override;
 
   // Adds PowerManagerClient, TimeZoneSettings and CrosSettings observers.
   void AddObservers() override;
@@ -127,6 +126,8 @@ class WallpaperManager :
   friend class WallpaperManagerBrowserTestDefaultWallpaper;
   friend class WallpaperManagerPolicyTest;
 
+  WallpaperManager();
+
   // Returns modifiable PendingWallpaper.
   // Returns pending_inactive_ or creates new PendingWallpaper if necessary.
   PendingWallpaper* GetPendingWallpaper(const std::string& user_id,