Move ImportantFileWriter to base/. It now has two consumers, base/prefs and chrome/.

chrome/chrome_tests.gypi

 # Copyright (c) 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.
 {
   'variables' : {
     'pyautolib_sources': [
       'app/chrome_command_ids.h',
       'app/chrome_dll_resource.h',
       'common/automation_constants.h',
       'common/pref_names.cc',
@@ skipping 2097 matching lines @@
         'common/extensions/permissions/api_permission_set_unittest.cc',
         'common/extensions/permissions/permission_set_unittest.cc',
         'common/extensions/permissions/socket_permission_unittest.cc',
         'common/extensions/unpacker_unittest.cc',
         'common/extensions/update_manifest_unittest.cc',
         'common/extensions/url_pattern_set_unittest.cc',
         'common/extensions/url_pattern_unittest.cc',
         'common/extensions/user_script_unittest.cc',
         'common/extensions/value_counter_unittest.cc',
         'common/extensions/api/extension_api_unittest.cc',
-        'common/important_file_writer_unittest.cc',
         'common/json_schema_validator_unittest.cc',
         'common/json_schema_validator_unittest_base.cc',
         'common/json_schema_validator_unittest_base.h',
         'common/json_value_serializer_unittest.cc',
         'common/mac/cfbundle_blocker_unittest.mm',
         'common/mac/mock_launchd.cc',
         'common/mac/mock_launchd.h',
         'common/mac/nscoder_util_unittest.mm',
         'common/mac/objc_method_swizzle_unittest.mm',
         'common/mac/objc_zombie_unittest.mm',
@@ skipping 2813 matching lines @@
             'sync_integration_tests.isolate',
           ],
           'sources': [
             'sync_integration_tests.isolate',
           ],
         },
       ],
     }],
   ],  # 'conditions'
 }