Add browser test for CustomizationWallpaperDownloader.

chrome/browser/chromeos/login/wallpaper_manager.h

Index: chrome/browser/chromeos/login/wallpaper_manager.h
diff --git a/chrome/browser/chromeos/login/wallpaper_manager.h b/chrome/browser/chromeos/login/wallpaper_manager.h
index e32280c538abc08b72f7e68554e19db9dde85738..73e8627fd308ef3769affbf5081a1dd3289b5085 100644
--- a/chrome/browser/chromeos/login/wallpaper_manager.h
+++ b/chrome/browser/chromeos/login/wallpaper_manager.h
@@ -121,6 +121,7 @@ class WallpaperManager: public content::NotificationObserver {
     virtual ~Observer() {}
     virtual void OnWallpaperAnimationFinished(const std::string& user_id) = 0;
     virtual void OnUpdateWallpaperForTesting() {}
+    virtual void OnPendingListEmptyForTesting() {}
   };
 
   // This is "wallpaper either scheduled to load, or loading right now".
@@ -479,6 +480,9 @@ class WallpaperManager: public content::NotificationObserver {
   PendingWallpaper* GetPendingWallpaper(const std::string& user_id,
                                         bool delayed);
 
+  // This is called by PendingWallpaper when load is finished.
+  void RemovePendingWallpaperFromList(PendingWallpaper* pending);
+
   // Calculate delay for next wallpaper load.
   // It is usually average wallpaper load time.
   // If last wallpaper load happened long ago, timeout should be reduced by