Auto merge of #18514 - asajeffrey:layout-dont-panic-if-no-iframe-bc, r=emilio

Remove sources of panic when laying out an iframe without a nested browsing context

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At the moment, layout panics if it discovers an iframe without a nested browsing context. Under normal circumstances, this is reasonable, but it requires very tight synchronization between script, layout, the constellation and the compositor. In particular, if a layout is in progress when an iframe's browsing context is discarded, this can trigger panic.

This PR fixes this in two ways:

1. Making the pipeline and browsing context ids optional in layout's representation of an iframe.
2. Shutting down layout before discarding a browsing context, rather than after.

This is belt and braces.

---
<!-- 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 #17482 and #18477
- [X] These changes do not require tests because the PR is fixing a panic caused by a race condition

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components/layout/display_list_builder.rs

@@ -2022,6 +2022,15 @@ impl FragmentDisplayListBuilding for Fragment {
             }
             SpecificFragmentInfo::Iframe(ref fragment_info) => {
                 if !stacking_relative_content_box.is_empty() {
+                    let browsing_context_id = match fragment_info.browsing_context_id {
+                        Some(browsing_context_id) => browsing_context_id,
+                        None => return warn!("No browsing context id for iframe."),
+                    };
+                    let pipeline_id = match fragment_info.pipeline_id {
+                        Some(pipeline_id) => pipeline_id,
+                        None => return warn!("No pipeline id for iframe {}.", browsing_context_id),
+                    };
+
                     let base = state.create_base_display_item(
                         &stacking_relative_content_box,
                         build_local_clip(&self.style),
@@ -2030,12 +2039,12 @@ impl FragmentDisplayListBuilding for Fragment {
                         DisplayListSection::Content);
                     let item = DisplayItem::Iframe(box IframeDisplayItem {
                         base: base,
-                        iframe: fragment_info.pipeline_id,
+                        iframe: pipeline_id,
                     });
 
                     let size = Size2D::new(item.bounds().size.width.to_f32_px(),
                                            item.bounds().size.height.to_f32_px());
-                    state.iframe_sizes.push((fragment_info.browsing_context_id,
+                    state.iframe_sizes.push((browsing_context_id,
                                              TypedSize2D::from_untyped(&size)));
 
                     state.add_display_item(item);

components/layout/fragment.rs

@@ -488,10 +488,10 @@ impl ImageFragmentInfo {
 /// size of this iframe can be communicated via the constellation to the iframe's own layout thread.
 #[derive(Clone)]
 pub struct IframeFragmentInfo {
-    /// The frame ID of this iframe.
-    pub browsing_context_id: BrowsingContextId,
-    /// The pipelineID of this iframe.
-    pub pipeline_id: PipelineId,
+    /// The frame ID of this iframe. None if there is no nested browsing context.
+    pub browsing_context_id: Option<BrowsingContextId>,
+    /// The pipelineID of this iframe. None if there is no nested browsing context.
+    pub pipeline_id: Option<PipelineId>,
 }
 
 impl IframeFragmentInfo {