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

base/base.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.
 
 {
   'target_defaults': {
     'variables': {
       'base_target': 0,
     },
     'target_conditions': [
@@ skipping 128 matching lines @@
           'file_version_info_win.h',
           'files/dir_reader_fallback.h',
           'files/dir_reader_linux.h',
           'files/dir_reader_posix.h',
           'files/file_path_watcher.cc',
           'files/file_path_watcher.h',
           'files/file_path_watcher_kqueue.cc',
           'files/file_path_watcher_linux.cc',
           'files/file_path_watcher_stub.cc',
           'files/file_path_watcher_win.cc',
+          'files/important_file_writer.h',
+          'files/important_file_writer.cc',
           'float_util.h',
           'format_macros.h',
           'global_descriptors_posix.cc',
           'global_descriptors_posix.h',
           'gtest_prod_util.h',
           'guid.cc',
           'guid.h',
           'guid_posix.cc',
           'guid_win.cc',
           'hash.cc',
@@ skipping 546 matching lines @@
           # Remove all unnecessary files for build_nexe.py to avoid exceeding
           # command-line-string limitation when building NaCl on Windows.
           ['OS == "win" and >(nacl_untrusted_build)==1', {
               'sources/': [ ['exclude', '\\.h$'] ],
           }],
         ],
       }],
     ],
   },
 }