Test file errors in downloads.

content/browser/download/download_file_manager.h

Index: content/browser/download/download_file_manager.h
diff --git a/content/browser/download/download_file_manager.h b/content/browser/download/download_file_manager.h
index b5f1654c4a74bf5323f64988d9fc932bb992fe76..2dd4a9f0d5b57582591aad620f85e312b3b8462f 100644
--- a/content/browser/download/download_file_manager.h
+++ b/content/browser/download/download_file_manager.h
@@ -148,10 +148,16 @@ class CONTENT_EXPORT DownloadFileManager
   friend class base::RefCountedThreadSafe<DownloadFileManager>;
   friend class DownloadFileManagerTest;
   friend class DownloadManagerTest;
+  friend class TestFileErrorInjectorImpl;
   FRIEND_TEST_ALL_PREFIXES(DownloadManagerTest, StartDownload);
 
   ~DownloadFileManager();
 
+  // This is only for unit tests.
+  void SetFileFactory(DownloadFileFactory* file_factory) {
+    download_file_factory_.reset(file_factory);
+  }
+
   // Timer helpers for updating the UI about the current progress of a download.
   void StartUpdateTimer();
   void StopUpdateTimer();