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.

---
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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 #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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1 files changed, 4 insertions, 4 deletions

diff --git a/components/script_layout_interface/wrapper_traits.rs b/components/script_layout_interface/wrapper_traits.rs
index c58b6292ef0..d022360305b 100644
--- a/components/script_layout_interface/wrapper_traits.rs
+++ b/components/script_layout_interface/wrapper_traits.rs
@@ -275,12 +275,12 @@ pub trait ThreadSafeLayoutNode: Clone + Copy + Debug + GetLayoutData + NodeInfo
     fn svg_data(&self) -> Option<SVGSVGData>;
 
     /// If this node is an iframe element, returns its browsing context ID. If this node is
-    /// not an iframe element, fails.
-    fn iframe_browsing_context_id(&self) -> BrowsingContextId;
+    /// not an iframe element, fails. Returns None if there is no nested browsing context.
+    fn iframe_browsing_context_id(&self) -> Option<BrowsingContextId>;
 
     /// If this node is an iframe element, returns its pipeline ID. If this node is
-    /// not an iframe element, fails.
-    fn iframe_pipeline_id(&self) -> PipelineId;
+    /// not an iframe element, fails. Returns None if there is no nested browsing context.
+    fn iframe_pipeline_id(&self) -> Option<PipelineId>;
 
     fn get_colspan(&self) -> u32;