Update the remaining include paths of base/string_piece.h to its new location.

crypto/p224_spake.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.
 
 #ifndef CRYPTO_P224_SPAKE_H_
 #define CRYPTO_P224_SPAKE_H_
 
-#include <base/string_piece.h>
+#include <base/strings/string_piece.h>
 #include <crypto/p224.h>
 #include <crypto/sha2.h>
 
 namespace crypto {
 
 // P224EncryptedKeyExchange implements SPAKE2, a variant of Encrypted
 // Key Exchange. It allows two parties that have a secret common
 // password to establish a common secure key by exchanging messages
 // over unsecure channel without disclosing the password.
 //
@@ skipping 86 matching lines @@
   // expected_authenticator_ is used to store the hash value expected from the
   // other party.
   uint8 expected_authenticator_[kSHA256Length];
 
   std::string key_;
 };
 
 }  // namespace crypto
 
 #endif  // CRYPTO_P224_SPAKE_H_