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, 6 insertions, 6 deletions

diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index 6b16cc242db..77e499a6f5a 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -1024,8 +1024,8 @@ pub trait LayoutNodeHelpers {
     fn image_url(&self) -> Option<ServoUrl>;
     fn canvas_data(&self) -> Option<HTMLCanvasData>;
     fn svg_data(&self) -> Option<SVGSVGData>;
-    fn iframe_browsing_context_id(&self) -> BrowsingContextId;
-    fn iframe_pipeline_id(&self) -> PipelineId;
+    fn iframe_browsing_context_id(&self) -> Option<BrowsingContextId>;
+    fn iframe_pipeline_id(&self) -> Option<PipelineId>;
     fn opaque(&self) -> OpaqueNode;
 }
 
@@ -1175,16 +1175,16 @@ impl LayoutNodeHelpers for LayoutJS<Node> {
             .map(|svg| svg.data())
     }
 
-    fn iframe_browsing_context_id(&self) -> BrowsingContextId {
+    fn iframe_browsing_context_id(&self) -> Option<BrowsingContextId> {
         let iframe_element = self.downcast::<HTMLIFrameElement>()
             .expect("not an iframe element!");
-        iframe_element.browsing_context_id().unwrap()
+        iframe_element.browsing_context_id()
     }
 
-    fn iframe_pipeline_id(&self) -> PipelineId {
+    fn iframe_pipeline_id(&self) -> Option<PipelineId> {
         let iframe_element = self.downcast::<HTMLIFrameElement>()
             .expect("not an iframe element!");
-        iframe_element.pipeline_id().unwrap()
+        iframe_element.pipeline_id()
     }
 
     #[allow(unsafe_code)]