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

content/browser/download/download_file.cc

Index: content/browser/download/download_file.cc
diff --git a/content/browser/download/download_file.cc b/content/browser/download/download_file.cc
index 06f643ef08f085a9fe45ec568cff6f1bedcd5920..8b5c9e6bd04bb62446e9fd18be0a5081e4bcdf62 100644
--- a/content/browser/download/download_file.cc
+++ b/content/browser/download/download_file.cc
@@ -25,7 +25,7 @@ static const int kMaxUniqueFiles = 100;
 
 DownloadFile::DownloadFile(const DownloadCreateInfo* info,
                            const DownloadRequestHandle& request_handle,
-                           DownloadManager* download_manager)
+                           DownloadManagerInterface* download_manager)
     : BaseFile(info->save_info.file_path,
                info->url(),
                info->referrer_url,
@@ -46,7 +46,7 @@ void DownloadFile::CancelDownloadRequest() {
   request_handle_.CancelRequest();
 }
 
-DownloadManager* DownloadFile::GetDownloadManager() {
+DownloadManagerInterface* DownloadFile::GetDownloadManager() {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
   return download_manager_.get();
 }