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

chrome/browser/download/chrome_download_manager_delegate.h

Index: chrome/browser/download/chrome_download_manager_delegate.h
diff --git a/chrome/browser/download/chrome_download_manager_delegate.h b/chrome/browser/download/chrome_download_manager_delegate.h
index fe9a1ccd85743fc9895da7439734b6b85c76afa0..f28d28412f19b6b63fd1897c9bded8364a738cd4 100644
--- a/chrome/browser/download/chrome_download_manager_delegate.h
+++ b/chrome/browser/download/chrome_download_manager_delegate.h
@@ -19,7 +19,7 @@
 class CrxInstaller;
 class DownloadHistory;
 class DownloadItem;
-class DownloadManager;
+class DownloadManagerInterface;
 class DownloadPrefs;
 class Profile;
 struct DownloadStateInfo;
@@ -45,7 +45,7 @@ class ChromeDownloadManagerDelegate
  public:
   explicit ChromeDownloadManagerDelegate(Profile* profile);
 
-  void SetDownloadManager(DownloadManager* dm);
+  void SetDownloadManager(DownloadManagerInterface* dm);
 
   // Returns true if the given item is for an extension download.
   static bool IsExtensionDownload(const DownloadItem* item);
@@ -90,7 +90,7 @@ class ChromeDownloadManagerDelegate
 
   // So that test classes that inherit from this for override purposes
   // can call back into the DownloadManager.
-  scoped_refptr<DownloadManager> download_manager_;
+  scoped_refptr<DownloadManagerInterface> download_manager_;
 
  private:
   friend class base::RefCountedThreadSafe<ChromeDownloadManagerDelegate>;