Created a DownloadManager interface, 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 105 matching lines @@
     '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',
+    'browser/download/download_manager_impl.cc',
+    'browser/download/download_manager_impl.h',
     'browser/download/download_persistent_store_info.cc',
     'browser/download/download_persistent_store_info.h',
     'browser/download/download_request_handle.cc',
     'browser/download/download_request_handle.h',
     'browser/download/download_resource_handler.cc',
     'browser/download/download_resource_handler.h',
     'browser/download/download_state_info.cc',
     'browser/download/download_state_info.h',
     'browser/download/download_stats.cc',
     'browser/download/download_stats.h',
@@ skipping 644 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',
       ],
     }],
   ],
 }