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

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

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Allow mozbrowser events, in particular mozbrowsererror events, to target a window. Needed for https://github.com/browserhtml/browserhtml/issues/1182

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- [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.

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components/script_traits/lib.rs

@@ -178,8 +178,9 @@ pub enum ConstellationControlMsg {
     NotifyVisibilityChange(PipelineId, PipelineId, bool),
     /// Notifies script thread that a url should be loaded in this iframe.
     Navigate(PipelineId, SubpageId, LoadData),
-    /// Requests the script thread forward a mozbrowser event to an iframe it owns
-    MozBrowserEvent(PipelineId, SubpageId, MozBrowserEvent),
+    /// Requests the script thread forward a mozbrowser event to an iframe it owns,
+    /// or to the window if no subpage id is provided.
+    MozBrowserEvent(PipelineId, Option<SubpageId>, MozBrowserEvent),
     /// Updates the current subpage and pipeline IDs of a given iframe
     UpdateSubpageId(PipelineId, SubpageId, SubpageId, PipelineId),
     /// Set an iframe to be focused. Used when an element in an iframe gains focus.