Auto merge of #18370 - pylbrecht:observer, r=ferjm

An observer disconnected after a mark must receive the mark

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I worked on top of #18283 as suggested [here](https://github.com/servo/servo/issues/18284#issuecomment-326043873).

r? @ferjm

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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 #18284 (github issue number if applicable).

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- [X] There are tests for these changes

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components/script/dom/performance.rs

@@ -182,6 +182,15 @@ impl Performance {
             Some(p) => p,
             None => return,
         };
+
+        if self.pending_notification_observers_task.get() {
+            if let Some(o) = observers.iter().nth(index) {
+                DOMPerformanceObserver::new(&self.global(),
+                                            o.observer.callback(),
+                                            o.observer.entries()).notify();
+            }
+        }
+
         observers.remove(index);
     }