Created an interface for DownloadFile, for use in unit tests.

content/browser/download/save_file.cc

 // 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.
 
 #include "content/browser/download/save_file.h"
 
 #include "base/logging.h"
 #include "content/public/browser/browser_thread.h"
 #include "net/base/file_stream.h"
 
 using content::BrowserThread;
 
 SaveFile::SaveFile(const SaveFileCreateInfo* info)
-    : BaseFile(FilePath(), info->url, GURL(), 0, linked_ptr<net::FileStream>()),
+    : file_(FilePath(), info->url, GURL(), 0, linked_ptr<net::FileStream>()),
       info_(info) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
 
   DCHECK(info);
   DCHECK(info->path.empty());
 }
 
 SaveFile::~SaveFile() {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
 }