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

base/base64.h

 // Copyright (c) 2011 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 BASE_BASE64_H__
 #define BASE_BASE64_H__
 
 #include <string>
 
 #include "base/base_export.h"
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 
 namespace base {
 
 // Encodes the input string in base64.  Returns true if successful and false
 // otherwise.  The output string is only modified if successful.
 BASE_EXPORT bool Base64Encode(const StringPiece& input, std::string* output);
 
 // Decodes the base64 input string.  Returns true if successful and false
 // otherwise.  The output string is only modified if successful.
 BASE_EXPORT bool Base64Decode(const StringPiece& input, std::string* output);
 
 }  // namespace base
 
 #endif  // BASE_BASE64_H__