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

base/strings/string_piece.h

Index: base/strings/string_piece.h
diff --git a/base/strings/string_piece.h b/base/strings/string_piece.h
index eaec14de5da788ebab2137df35015b9998ec892e..5333640fee3399441a62439011679f402824c6b1 100644
--- a/base/strings/string_piece.h
+++ b/base/strings/string_piece.h
@@ -245,6 +245,9 @@ template <typename STRING_TYPE> class BasicStringPiece {
     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());