Rewrite DownloadHandlerTest using DummyFileSystem.

chrome/chrome_tests_unit.gypi

 # Copyright 2012 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.
 {
   'targets': [
     {
       # This target contains mocks and test utilities that don't belong in
       # production libraries but are used by more than one test executable.
       'target_name': 'test_support_common',
       'type': 'static_library',
@@ skipping 584 matching lines @@
         'browser/chromeos/drive/file_system/update_operation_unittest.cc',
         'browser/chromeos/drive/file_system_util_unittest.cc',
         'browser/chromeos/drive/file_write_helper_unittest.cc',
         'browser/chromeos/drive/job_queue_unittest.cc',
         'browser/chromeos/drive/job_scheduler_unittest.cc',
         'browser/chromeos/drive/local_file_reader_unittest.cc',
         'browser/chromeos/drive/mock_directory_change_observer.cc',
         'browser/chromeos/drive/mock_directory_change_observer.h',
         'browser/chromeos/drive/mock_file_cache_observer.cc',
         'browser/chromeos/drive/mock_file_cache_observer.h',
-        'browser/chromeos/drive/mock_file_system.cc',
-        'browser/chromeos/drive/mock_file_system.h',
         'browser/chromeos/drive/remove_stale_cache_files_unittest.cc',
         'browser/chromeos/drive/resource_entry_conversion_unittest.cc',
         'browser/chromeos/drive/resource_metadata_storage_unittest.cc',
         'browser/chromeos/drive/resource_metadata_unittest.cc',
         'browser/chromeos/drive/search_metadata_unittest.cc',
         'browser/chromeos/drive/sync_client_unittest.cc',
         'browser/chromeos/drive/test_util.cc',
         'browser/chromeos/drive/test_util.h',
         'browser/chromeos/drive/webkit_file_stream_reader_impl_unittest.cc',
         'browser/chromeos/extensions/default_app_order_unittest.cc',
@@ skipping 1936 matching lines @@
               # more details.
               'DebugInformationFormat': '3',
               'ProgramDataBaseFileName': '<(PRODUCT_DIR)/unit_tests.exe.pdb',
             },
           },
         },
       ],
     }],
   ],  # 'conditions'
 }