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

base/md5.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_MD5_H_
 #define BASE_MD5_H_
 
 #include "base/base_export.h"
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 
 namespace base {
 
 // MD5 stands for Message Digest algorithm 5.
 // MD5 is a robust hash function, designed for cyptography, but often used
 // for file checksums.  The code is complex and slow, but has few
 // collisions.
 // See Also:
 //   http://en.wikipedia.org/wiki/MD5
 
@@ skipping 40 matching lines @@
 
 // Converts a digest into human-readable hexadecimal.
 BASE_EXPORT std::string MD5DigestToBase16(const MD5Digest& digest);
 
 // Returns the MD5 (in hexadecimal) of a string.
 BASE_EXPORT std::string MD5String(const StringPiece& str);
 
 }  // namespace base
 
 #endif  // BASE_MD5_H_