Auto merge of #11585 - asajeffrey:constellation-avoid-deadlock-during-pipeline-closure, r=larsbergstrom

Avoid deadlock when closing a pipeline.

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At the moment, the constellation blocks on a pipeline during closure. This PR makes pipeline closure asynchronous.

---
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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 #11546.
- [X] These changes do not require tests because testing for absence of deadlock is difficult.

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -6472,6 +6472,12 @@
             "url": "/_mozilla/mozilla/iframe_hierarchy.html"
           }
         ],
+        "mozilla/iframe_replacement.html": [
+          {
+            "path": "mozilla/iframe_replacement.html",
+            "url": "/_mozilla/mozilla/iframe_replacement.html"
+          }
+        ],
         "mozilla/img_width_height.html": [
           {
             "path": "mozilla/img_width_height.html",