During the https://bugs.webkit.org/show_bug.cgi?id=155602 it was noted that Windows, GTK, and EFL call ResourceResponseBase::addHTTPHeaderField() with a String reference as the header name. We should refactor these pieces to instead use HTTPHeaderName::findHTTPHeaderName() and send a HTTPHeaderName enum value to esourceResponseBase::addHTTPHeaderField(). Thus, unknown header names can be handled where error handling makes more sense. The same goes for ResourceRequestBase::addHTTPHeaderField(). After this refactoring it should be possible to remove the addHTTPHeaderField() functions that receive HTTP header names as strings.