LibWeb: Port Element::get_attribute_value from DeprecatedFlyString

Userland/Libraries/LibWeb/DOM/Element.cpp

@@ -155,7 +155,7 @@ DeprecatedString Element::deprecated_get_attribute(StringView name) const
 }
 
 // https://dom.spec.whatwg.org/#concept-element-attributes-get-value
-DeprecatedString Element::get_attribute_value(StringView local_name, DeprecatedFlyString const& namespace_) const
+DeprecatedString Element::get_attribute_value(FlyString const& local_name, Optional<FlyString> const& namespace_) const
 {
     // 1. Let attr be the result of getting an attribute given namespace, localName, and element.
     auto const* attribute = m_attributes->get_attribute_ns(namespace_, local_name);

Userland/Libraries/LibWeb/DOM/Element.h

@@ -103,7 +103,7 @@ public:
     // FIXME: This should be taking a 'FlyString const&' / 'Optional<FlyString> const&'
     Optional<String> get_attribute(StringView name) const;
     DeprecatedString deprecated_get_attribute(StringView name) const;
-    DeprecatedString get_attribute_value(StringView local_name, DeprecatedFlyString const& namespace_ = {}) const;
+    DeprecatedString get_attribute_value(FlyString const& local_name, Optional<FlyString> const& namespace_ = {}) const;
 
     WebIDL::ExceptionOr<void> set_attribute(FlyString const& name, String const& value);