Created a DownloadManager interface, for use in unit tests..

chrome/browser/download/download_prefs.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_DOWNLOAD_DOWNLOAD_PREFS_H_
 #define CHROME_BROWSER_DOWNLOAD_DOWNLOAD_PREFS_H_
 #pragma once
 
 #include <set>
 
 #include "base/file_path.h"
 #include "chrome/browser/prefs/pref_member.h"
 
-class DownloadManager;
+class DownloadManagerInterface;
 class PrefService;
 
 // Stores all download-related preferences.
 class DownloadPrefs {
  public:
   explicit DownloadPrefs(PrefService* prefs);
   ~DownloadPrefs();
 
   static void RegisterUserPrefs(PrefService* prefs);
 
   // Returns the DownloadPrefs corresponding to the given DownloadManager.
-  static DownloadPrefs* FromDownloadManager(DownloadManager* download_manager);
+  static DownloadPrefs* FromDownloadManager(
+      DownloadManagerInterface* download_manager);
 
   FilePath download_path() const { return *download_path_; }
   int save_file_type() const { return *save_file_type_; }
 
   // Returns true if the prompt_for_download preference has been set and the
   // download location is not managed (which means the user shouldn't be able
   // to choose another download location).
   bool PromptForDownload() const;
 
   // Returns true if the download path preference is managed.
@@ skipping 29 matching lines @@
     bool operator()(const FilePath::StringType& a,
                     const FilePath::StringType& b) const;
   };
   typedef std::set<FilePath::StringType, AutoOpenCompareFunctor> AutoOpenSet;
   AutoOpenSet auto_open_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadPrefs);
 };
 
 #endif  // CHROME_BROWSER_DOWNLOAD_DOWNLOAD_PREFS_H_