servo: Merge #18370 - An observer disconnected after a mark must receive the mark (from pylbrecht:observer); r=ferjm

<!-- Please describe your changes on the following line: -->
I worked on top of #18283 as suggested [here](https://github.com/servo/servo/issues/18284#issuecomment-326043873).

r? @ferjm

<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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).

<!-- Either: -->
- [X] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

Source-Repo: https://github.com/servo/servo
Source-Revision: 867d542261336d918177f8780ef915b7ec84e3ba

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 79691b6f8e710beeab4ca8ba260f4ee0869914a7

servo/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);
     }