Use a direct include of strings headers in base/.

base/i18n/string_search.h

 // 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.
 
 #ifndef BASE_I18N_STRING_SEARCH_H_
 #define BASE_I18N_STRING_SEARCH_H_
 
 #include "base/i18n/base_i18n_export.h"
-#include "base/string16.h"
+#include "base/strings/string16.h"
 
 namespace base {
 namespace i18n {
 
 // Returns true if |in_this| contains |find_this|. If |match_index| or
 // |match_length| are non-NULL, they are assigned the start position and total
 // length of the match.
 //
 // Only differences between base letters are taken into consideration. Case and
 // accent differences are ignored. Please refer to 'primary level' in
 // http://userguide.icu-project.org/collation/concepts for additional details.
 BASE_I18N_EXPORT
     bool StringSearchIgnoringCaseAndAccents(const string16& find_this,
                                             const string16& in_this,
                                             size_t* match_index,
                                             size_t* match_length);
 
 }  // namespace i18n
 }  // namespace base
 
 #endif  // BASE_I18N_STRING_SEARCH_H_