Move nullable_string16.h to the string subdirectory.

base/nullable_string16.h

 // Copyright (c) 2010 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_NULLABLE_STRING16_H_
-#define BASE_NULLABLE_STRING16_H_
-
-#include "base/strings/string16.h"
-
-// This class is a simple wrapper for string16 which also contains a null
-// state.  This should be used only where the difference between null and
-// empty is meaningful.
-class NullableString16 {
- public:
-  NullableString16() : is_null_(false) { }
-  explicit NullableString16(bool is_null) : is_null_(is_null) { }
-  NullableString16(const string16& string, bool is_null)
-      : string_(string), is_null_(is_null) {
-  }
-
-  const string16& string() const { return string_; }
-  bool is_null() const { return is_null_; }
-
- private:
-  string16 string_;
-  bool is_null_;
-};
-
-#endif  // BASE_NULLABLE_STRING16_H_
+// This file has moved, please use the new location.
+// TODO(avi) remove this file when all users have been updated.
+#include "base/strings/nullable_string16.h"