Use a direct include of strings headers in base/.

base/strings/utf_string_conversion_utils.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_STRINGS_UTF_STRING_CONVERSION_UTILS_H_
 #define BASE_STRINGS_UTF_STRING_CONVERSION_UTILS_H_
 
 // This should only be used by the various UTF string conversion files.
 
 #include "base/base_export.h"
-#include "base/string16.h"
+#include "base/strings/string16.h"
 
 namespace base {
 
 inline bool IsValidCodepoint(uint32 code_point) {
   // Excludes the surrogate code points ([0xD800, 0xDFFF]) and
   // codepoints larger than 0x10FFFF (the highest codepoint allowed).
   // Non-characters and unassigned codepoints are allowed.
   return code_point < 0xD800u ||
          (code_point >= 0xE000u && code_point <= 0x10FFFFu);
 }
@@ skipping 66 matching lines @@
 void PrepareForUTF8Output(const CHAR* src, size_t src_len, std::string* output);
 
 // Prepares an output buffer (containing either UTF-16 or -32 data) given some
 // UTF-8 input that will be converted to it.  See PrepareForUTF8Output().
 template<typename STRING>
 void PrepareForUTF16Or32Output(const char* src, size_t src_len, STRING* output);
 
 }  // namespace base
 
 #endif  // BASE_STRINGS_UTF_STRING_CONVERSION_UTILS_H_