Auto merge of #16506 - cbrewster:about_chaos, r=asajeffrey

Make non-initial about:blank loads async

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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 #14856 (github issue number if applicable).

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components/script_layout_interface/wrapper_traits.rs

@@ -11,7 +11,7 @@ use SVGSVGData;
 use atomic_refcell::AtomicRefCell;
 use gfx_traits::{ByteIndex, FragmentType, combine_id_with_fragment_type};
 use html5ever::{Namespace, LocalName};
-use msg::constellation_msg::PipelineId;
+use msg::constellation_msg::{FrameId, PipelineId};
 use range::Range;
 use servo_url::ServoUrl;
 use std::fmt::Debug;
@@ -271,6 +271,10 @@ pub trait ThreadSafeLayoutNode: Clone + Copy + Debug + GetLayoutData + NodeInfo
 
     fn svg_data(&self) -> Option<SVGSVGData>;
 
+    /// If this node is an iframe element, returns its frame ID. If this node is
+    /// not an iframe element, fails.
+    fn iframe_frame_id(&self) -> FrameId;
+
     /// 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;