Enable wallpaper boot animation for boot into sign in too. Add flag to disable it.

ash/desktop_background/desktop_background_controller.h

Index: ash/desktop_background/desktop_background_controller.h
diff --git a/ash/desktop_background/desktop_background_controller.h b/ash/desktop_background/desktop_background_controller.h
index 813e338266e25e08d919be8a2ecc502359f3908d..ceb3868fc27aa8e4b75d5e6bb8d07ed08da2aa9c 100644
--- a/ash/desktop_background/desktop_background_controller.h
+++ b/ash/desktop_background/desktop_background_controller.h
@@ -40,6 +40,9 @@ class UserWallpaperDelegate {
 
   // Notifies delegate that wallpaper animation has finished.
   virtual void OnWallpaperAnimationFinished() = 0;
+
+  // Notifies delegate that wallpaper boot animation has finished.
+  virtual void OnWallpaperBootAnimationFinished() = 0;
 };
 
 // Loads selected desktop wallpaper from file system asynchronously and updates