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

ash/display/display_pref_util.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 ASH_DISPLAY_DISPLAY_PREF_UTIL_H
 #define ASH_DISPLAY_DISPLAY_PREF_UTIL_H
 
 #include <map>
 #include <string>
 
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
 
 namespace ash {
 
 // Utility templates to create enum to string map and
 // a function to find an enum value from a string.
 template<typename T>
 std::map<T, std::string>* CreateToStringMap(T k1, const std::string& v1,
                                             T k2, const std::string& v2,
                                             T k3, const std::string& v3,
                                             T k4, const std::string& v4) {
@@ skipping 17 matching lines @@
       *key = iter->first;
       return true;
     }
   }
   return false;
 }
 
 }  // namespace ash
 
 #endif  // ASH_DISPLAY_DISPLAY_PREF_UTIL_H