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/customevent.rs

@@ -93,7 +93,7 @@ impl CustomEventMethods<crate::DomTypeHolder> for CustomEvent {
         type_: DOMString,
         init: RootedTraceableBox<CustomEventBinding::CustomEventInit>,
     ) -> DomRoot<CustomEvent> {
-        CustomEvent::new(
+        let event = CustomEvent::new(
             global,
             proto,
             Atom::from(type_),
@@ -101,7 +101,9 @@ impl CustomEventMethods<crate::DomTypeHolder> for CustomEvent {
             init.parent.cancelable,
             init.detail.handle(),
             can_gc,
-        )
+        );
+        event.upcast::<Event>().set_composed(init.parent.composed);
+        event
     }
 
     // https://dom.spec.whatwg.org/#dom-customevent-detail