Include <wchar.h> for mbstate_t.

base/strings/string16.h

 // Copyright 2013 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_STRING16_H_
 #define BASE_STRINGS_STRING16_H_
 
 // WHAT:
 // A version of std::basic_string that provides 2-byte characters even when
 // wchar_t is not implemented as a 2-byte type. You can access this class as
@@ skipping 30 matching lines @@
 namespace base {
 
 typedef wchar_t char16;
 typedef std::wstring string16;
 typedef std::char_traits<wchar_t> string16_char_traits;
 
 }  // namespace base
 
 #elif defined(WCHAR_T_IS_UTF32)
 
+#include <wchar.h>  // for mbstate_t
+
 namespace base {
 
 typedef uint16_t char16;
 
 // char16 versions of the functions required by string16_char_traits; these
 // are based on the wide character functions of similar names ("w" or "wcs"
 // instead of "c16").
 BASE_EXPORT int c16memcmp(const char16* s1, const char16* s2, size_t n);
 BASE_EXPORT size_t c16len(const char16* s);
 BASE_EXPORT const char16* c16memchr(const char16* s, char16 c, size_t n);
@@ skipping 134 matching lines @@
     for (base::char16 c : s)
       result = (result * 131) + c;
     return result;
   }
 };
 }  // namespace std
 
 #endif  // WCHAR_T_IS_UTF32
 
 #endif  // BASE_STRINGS_STRING16_H_