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

content/browser/download/download_file.h

Index: content/browser/download/download_file.h
diff --git a/content/browser/download/download_file.h b/content/browser/download/download_file.h
index d7795c716e1a4d38122e00d23e08a36b2b536f85..d5b04260e293d497a32045ea82d7ea0ec0760f92 100644
--- a/content/browser/download/download_file.h
+++ b/content/browser/download/download_file.h
@@ -17,7 +17,7 @@
 #include "content/common/content_export.h"
 
 struct DownloadCreateInfo;
-class DownloadManager;
+class DownloadManagerInterface;
 class ResourceDispatcherHost;
 
 // These objects live exclusively on the download thread and handle the writing
@@ -28,14 +28,14 @@ class CONTENT_EXPORT DownloadFile : public BaseFile {
  public:
   DownloadFile(const DownloadCreateInfo* info,
                const DownloadRequestHandle& request_handle,
-               DownloadManager* download_manager);
+               DownloadManagerInterface* download_manager);
   virtual ~DownloadFile();
 
   // Cancels the download request associated with this file.
   void CancelDownloadRequest();
 
   int id() const { return id_.local(); }
-  DownloadManager* GetDownloadManager();
+  DownloadManagerInterface* GetDownloadManager();
   const DownloadId& global_id() const { return id_; }
 
   virtual std::string DebugString() const;
@@ -70,7 +70,7 @@ class CONTENT_EXPORT DownloadFile : public BaseFile {
   DownloadRequestHandle request_handle_;
 
   // DownloadManager this download belongs to.
-  scoped_refptr<DownloadManager> download_manager_;
+  scoped_refptr<DownloadManagerInterface> download_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadFile);
 };