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

crypto/hkdf.h

 // Copyright (c) 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 CRYPTO_HKDF_H_
 #define CRYPTO_HKDF_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 #include "build/build_config.h"
 #include "crypto/crypto_export.h"
 
 namespace crypto {
 
 // HKDF implements the key derivation function specified in RFC 5869 (using
 // SHA-256) and outputs key material, as needed by QUIC.
 // See https://tools.ietf.org/html/rfc5869 for details.
 class CRYPTO_EXPORT HKDF {
  public:
@@ skipping 32 matching lines @@
 
   base::StringPiece client_write_key_;
   base::StringPiece server_write_key_;
   base::StringPiece client_write_iv_;
   base::StringPiece server_write_iv_;
 };
 
 }  // namespace crypto
 
 #endif  // CRYPTO_HKDF_H_