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

content/content_browser.gypi

 # 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.
 
 {
   'dependencies': [
     '../crypto/crypto.gyp:crypto',
     '../net/net.gyp:net',
     '../net/net.gyp:http_server',
     '../ppapi/ppapi_internal.gyp:ppapi_proxy',
@@ skipping 95 matching lines @@
     'browser/disposition_utils.cc',
     'browser/disposition_utils.h',
     'browser/download/base_file.cc',
     'browser/download/base_file.h',
     'browser/download/download_buffer.cc',
     'browser/download/download_buffer.h',
     'browser/download/download_create_info.cc',
     'browser/download/download_create_info.h',
     'browser/download/download_file.cc',
     'browser/download/download_file.h',
+    'browser/download/download_file_impl.cc',
+    'browser/download/download_file_impl.h',
     'browser/download/download_file_manager.cc',
     'browser/download/download_file_manager.h',
     'browser/download/download_id.cc',
     'browser/download/download_id.h',
     'browser/download/download_id_factory.cc',
     'browser/download/download_id_factory.h',
     'browser/download/download_item.cc',
     'browser/download/download_item.h',
     'browser/download/download_manager.cc',
     'browser/download/download_manager.h',
@@ skipping 653 matching lines @@
         'browser/renderer_host/java/java_bridge_dispatcher_host_manager.h',
         'browser/renderer_host/java/java_bridge_dispatcher_host.cc',
         'browser/renderer_host/java/java_bridge_dispatcher_host.h',
         'browser/renderer_host/java/java_method.cc',
         'browser/renderer_host/java/java_method.h',
       ],
     }],
   ],
 }