Add browser test for CustomizationWallpaperDownloader.

chrome/browser/chromeos/customization_wallpaper_downloader.h

Index: chrome/browser/chromeos/customization_wallpaper_downloader.h
diff --git a/chrome/browser/chromeos/customization_wallpaper_downloader.h b/chrome/browser/chromeos/customization_wallpaper_downloader.h
index 9922ffbdc2b9a43285784a4b985910a767735f09..091c95778945a2d0adfb596c43bb482cddfea47b 100644
--- a/chrome/browser/chromeos/customization_wallpaper_downloader.h
+++ b/chrome/browser/chromeos/customization_wallpaper_downloader.h
@@ -52,6 +52,11 @@ class CustomizationWallpaperDownloader : public net::URLFetcherDelegate {
   // net::URLFetcherDelegate
   virtual void OnURLFetchComplete(const net::URLFetcher* source) OVERRIDE;
 
+  // This is called in tests to modify (lower) retry delay.
+  void set_retry_delay_for_testing(base::TimeDelta value) {
+    retry_delay_ = value;
+  }
+
  private:
   // Start new request.
   void StartRequest();
@@ -89,6 +94,10 @@ class CustomizationWallpaperDownloader : public net::URLFetcherDelegate {
   // Number of download retries (first attempt is not counted as retry).
   size_t retries_;
 
+  // Sleep between retry requests (increasing, see Retry() method for details).
+  // Non-constant value for tests.
+  base::TimeDelta retry_delay_;
+
   // Callback supplied by caller.
   base::Callback<void(bool success, const GURL&)> on_wallpaper_fetch_completed_;