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

crypto/sha2.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_SHA2_H_
 #define CRYPTO_SHA2_H_
 
 #include <string>
 
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 #include "crypto/crypto_export.h"
 
 namespace crypto {
 
 // These functions perform SHA-256 operations.
 //
 // Functions for SHA-384 and SHA-512 can be added when the need arises.
 
 static const size_t kSHA256Length = 32;  // Length in bytes of a SHA-256 hash.
 
 // Computes the SHA-256 hash of the input string 'str' and stores the first
 // 'len' bytes of the hash in the output buffer 'output'.  If 'len' > 32,
 // only 32 bytes (the full hash) are stored in the 'output' buffer.
 CRYPTO_EXPORT void SHA256HashString(const base::StringPiece& str,
                                     void* output, size_t len);
 
 // Convenience version of the above that returns the result in a 32-byte
 // string.
 CRYPTO_EXPORT std::string SHA256HashString(const base::StringPiece& str);
 
 }  // namespace crypto
 
 #endif  // CRYPTO_SHA2_H_