Auto merge of #14679 - jdm:nowarntimer, r=asajeffrey

Warn when a timer event is processed for a discarded document.

Turn a panic into a warning for a situation that is easy to hit in web content.

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14677
- [X] There are tests for these changes

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components/script/script_thread.rs

@@ -1049,8 +1049,11 @@ impl ScriptThread {
             TimerSource::FromWorker => panic!("Worker timeouts must not be sent to script thread"),
         };
 
-        let window = self.documents.borrow().find_window(pipeline_id)
-            .expect("ScriptThread: received fire timer msg for a pipeline not in this script thread. This is a bug.");
+        let window = self.documents.borrow().find_window(pipeline_id);
+        let window = match window {
+            Some(w) => w,
+            None => return warn!("Received fire timer msg for a closed pipeline {}.", pipeline_id),
+        };
 
         window.handle_fire_timer(id);
     }