Revert of Allow std::unordered_*.

base/strings/string_piece.h

Index: base/strings/string_piece.h
diff --git a/base/strings/string_piece.h b/base/strings/string_piece.h
index 92634b9362b7dfb405ef7c7210f90688e385d25d..31e7596d119c24b9c9a48e22178b7f4f62f1fa83 100644
--- a/base/strings/string_piece.h
+++ b/base/strings/string_piece.h
@@ -439,9 +439,9 @@
 // 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/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.
+// 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.
 #define HASH_STRING_PIECE(StringPieceType, string_piece)                \
   std::size_t result = 0;                                               \
   for (StringPieceType::const_iterator i = string_piece.begin();        \