Allow std::unordered_*.

base/strings/string_piece.h

 // 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.
 // Copied from strings/stringpiece.h with modifications
 //
 // A string-like object that points to a sized piece of memory.
 //
 // You can use StringPiece as a function or method parameter.  A StringPiece
 // parameter can receive a double-quoted string literal argument, a "const
 // char*" argument, a string argument, or a StringPiece argument with no data
@@ skipping 421 matching lines @@
 BASE_EXPORT std::ostream& operator<<(std::ostream& o,
                                      const StringPiece& piece);
 
 }  // namespace base
 
 // Hashing ---------------------------------------------------------------------
 
 // We provide appropriate hash functions so StringPiece and StringPiece16 can
 // be used as keys in hash sets and maps.
 
-// This hash function is copied from base/containers/hash_tables.h. We don't
-// use the ones already defined for string and string16 directly because it
-// would require the string constructors to be called, which we don't want.
+// This hash function is copied from base/strings/string16.h. We don't use the
+// ones already defined for string and string16 directly because it would
+// require the string constructors to be called, which we don't want.
 #define HASH_STRING_PIECE(StringPieceType, string_piece)                \
   std::size_t result = 0;                                               \
   for (StringPieceType::const_iterator i = string_piece.begin();        \
        i != string_piece.end(); ++i)                                    \
     result = (result * 131) + *i;                                       \
   return result;                                                        \
 
 namespace BASE_HASH_NAMESPACE {
 
 template<>
 struct hash<base::StringPiece> {
   std::size_t operator()(const base::StringPiece& sp) const {
     HASH_STRING_PIECE(base::StringPiece, sp);
   }
 };
 template<>
 struct hash<base::StringPiece16> {
   std::size_t operator()(const base::StringPiece16& sp16) const {
     HASH_STRING_PIECE(base::StringPiece16, sp16);
   }
 };
 
 }  // namespace BASE_HASH_NAMESPACE
 
 #endif  // BASE_STRINGS_STRING_PIECE_H_