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/constellation/pipeline.rs

@@ -379,7 +379,7 @@ impl Pipeline {
     }
 
     pub fn trigger_mozbrowser_event(&self,
-                                     subpage_id: SubpageId,
+                                     subpage_id: Option<SubpageId>,
                                      event: MozBrowserEvent) {
         assert!(PREFS.is_mozbrowser_enabled());