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

An observer disconnected after a mark must receive the mark

<!-- 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. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18370)
<!-- Reviewable:end -->

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);
     }
 

tests/wpt/metadata/performance-timeline/po-disconnect.any.js.ini

@@ -1,13 +0,0 @@
-[po-disconnect.any.html]
-  type: testharness
-  expected: TIMEOUT
-  [An observer disconnected after a mark must receive the mark]
-    expected: TIMEOUT
-
-
-[po-disconnect.any.worker.html]
-  type: testharness
-  expected: TIMEOUT
-  [An observer disconnected after a mark must receive the mark]
-    expected: TIMEOUT
-