dom: Set "composed" flag on construction for some event types (#36239)

"Composed" flag (https://dom.spec.whatwg.org/#composed-flag) should be
properly set on event construction phase from optional "EventInit"
dictionary
(https://dom.spec.whatwg.org/#dom-eventinit-composed).

The limited set of event types (Custom/Error/Focus/Mouse) will be
affected by this CL (used in WPT tests).

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

- [X] There are WPT shadow-dom tests which have new issues related to
event "relatedTarget" property
tests/wpt/tests/shadow-dom/event-composed-path-with-related-target.html
  tests/wpt/tests/shadow-dom/event-with-related-target.html

Signed-off-by: Andrei Volykhin <andrei.volykhin@gmail.com>

components/script/dom/focusevent.rs

@@ -15,7 +15,7 @@ use crate::dom::bindings::inheritance::Castable;
 use crate::dom::bindings::reflector::reflect_dom_object_with_proto;
 use crate::dom::bindings::root::{DomRoot, MutNullableDom};
 use crate::dom::bindings::str::DOMString;
-use crate::dom::event::{EventBubbles, EventCancelable};
+use crate::dom::event::{Event, EventBubbles, EventCancelable};
 use crate::dom::eventtarget::EventTarget;
 use crate::dom::uievent::UIEvent;
 use crate::dom::window::Window;
@@ -118,6 +118,9 @@ impl FocusEventMethods<crate::DomTypeHolder> for FocusEvent {
             init.relatedTarget.as_deref(),
             can_gc,
         );
+        event
+            .upcast::<Event>()
+            .set_composed(init.parent.parent.composed);
         Ok(event)
     }