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

chrome/browser/download/download_prefs.h

Index: chrome/browser/download/download_prefs.h
diff --git a/chrome/browser/download/download_prefs.h b/chrome/browser/download/download_prefs.h
index 848db7a542060462447a042ce25ad9fce133ef47..95c4fe32c57929222b815a57bfb4c2d4d685dbc9 100644
--- a/chrome/browser/download/download_prefs.h
+++ b/chrome/browser/download/download_prefs.h
@@ -11,7 +11,7 @@
 #include "base/file_path.h"
 #include "chrome/browser/prefs/pref_member.h"
 
-class DownloadManager;
+class DownloadManagerInterface;
 class PrefService;
 
 // Stores all download-related preferences.
@@ -23,7 +23,8 @@ class 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_; }