Use string(StringPiece) instead of StringPiece::as_string().

base/strings/string_piece.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.
 // Copied from strings/stringpiece.h with modifications
 //
 // A string-like object that points to a sized piece of memory.
 //
 // You can use StringPiece as a function or method parameter.  A StringPiece
 // parameter can receive a double-quoted string literal argument, a "const
 // char*" argument, a string argument, or a StringPiece argument with no data
@@ skipping 227 matching lines @@
   int compare(const BasicStringPiece<STRING_TYPE>& x) const {
     int r = wordmemcmp(
         ptr_, x.ptr_, (length_ < x.length_ ? length_ : x.length_));
     if (r == 0) {
       if (length_ < x.length_) r = -1;
       else if (length_ > x.length_) r = +1;
     }
     return r;
   }
 
+  // This is the style of conversion preferred by std::string_view in C++17.
+  explicit operator STRING_TYPE() const { return as_string(); }
+
   STRING_TYPE as_string() const {
     // std::string doesn't like to take a NULL pointer even with a 0 size.
     return empty() ? STRING_TYPE() : STRING_TYPE(data(), size());
   }
 
   const_iterator begin() const { return ptr_; }
   const_iterator end() const { return ptr_ + length_; }
   const_reverse_iterator rbegin() const {
     return const_reverse_iterator(ptr_ + length_);
   }
@@ skipping 197 matching lines @@
 };
 struct StringPiece16Hash {
   std::size_t operator()(const StringPiece16& sp16) const {
     HASH_STRING_PIECE(StringPiece16, sp16);
   }
 };
 
 }  // namespace base
 
 #endif  // BASE_STRINGS_STRING_PIECE_H_