Auto merge of #12426 - asajeffrey:mozbrowser-event-targets, r=SimonSapin

Allow window elements as well as iframes to the the target of mozbrowser events

<!-- Please describe your changes on the following line: -->
Allow mozbrowser events, in particular mozbrowsererror events, to target a window. Needed for https://github.com/browserhtml/browserhtml/issues/1182

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12420
- [X] These changes do not require tests because we're not testing our issue reporting system, which this is intended for.

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12426)
<!-- Reviewable:end -->

components/script_traits/script_msg.rs

@@ -88,8 +88,9 @@ pub enum ScriptMsg {
     LoadComplete(PipelineId),
     /// A new load has been requested.
     LoadUrl(PipelineId, LoadData),
-    /// Dispatch a mozbrowser event to a given iframe. Only available in experimental mode.
-    MozBrowserEvent(PipelineId, SubpageId, MozBrowserEvent),
+    /// Dispatch a mozbrowser event to a given iframe,
+    /// or to the window if no subpage id is provided.
+    MozBrowserEvent(PipelineId, Option<SubpageId>, MozBrowserEvent),
     /// HTMLIFrameElement Forward or Back navigation.
     Navigate(Option<(PipelineId, SubpageId)>, NavigationDirection),
     /// Favicon detected