Auto merge of #21388 - gterzian:introduce_task_queues, r=jdm

Introduce task queues, and throttling performance timeline tasksource

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #19997 (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- 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/21388)
<!-- Reviewable:end -->

tests/wpt/mozilla/meta/MANIFEST.json

@@ -20674,6 +20674,16 @@
      {}
     ]
    ],
+   "mozilla/task_queue_throttling.any.js": [
+    [
+     "/_mozilla/mozilla/task_queue_throttling.any.html",
+     {}
+    ],
+    [
+     "/_mozilla/mozilla/task_queue_throttling.any.worker.html",
+     {}
+    ]
+   ],
    "mozilla/textcontent.html": [
     [
      "/_mozilla/mozilla/textcontent.html",
@@ -32667,6 +32677,10 @@
    "321132b7e7e21d542ce86e4322db04aba3e9d11e",
    "support"
   ],
+  "mozilla/task_queue_throttling.any.js": [
+   "bbf464f384d55a44db6948d6cc7ab5f54509b800",
+   "testharness"
+  ],
   "mozilla/test.jpg": [
    "110b3d6b666d35a117ca8c8ada32bf1002943044",
    "support"

tests/wpt/mozilla/tests/mozilla/task_queue_throttling.any.js

@@ -0,0 +1,45 @@
+// META: title=Throttling the performance timeline task queue.
+
+async_test(function(t) {
+  var counter = 0;
+
+  function perf_observer(list, observer) {
+    // The timeline event should be throttled,
+    // while the event-loop is busy,
+    // and only handled after at least 6 other events,
+    // across several iterations of the event-loop.
+    assert_true(counter > 6)
+  }
+  var observer2 = new PerformanceObserver(t.step_func_done(perf_observer));
+  observer2.observe({entryTypes: ["mark"]});
+
+  for (var i = 0; i < 4; i++) {
+      var reader = new FileReader();
+      reader.onload = function() {
+         counter++;
+      };
+      var blob = new Blob();
+      reader.readAsText(blob);
+  }
+
+  var reader = new FileReader();
+  reader.onload = function() {
+     counter++;
+     // In a subsequent iteration of the event-loop,
+     // start reading another 5 blobs
+     for (var i = 0; i < 5; i++) {
+        var reader = new FileReader();
+        reader.onload = function() {
+           counter++;
+        };
+        var blob = new Blob();
+        reader.readAsText(blob);
+     }
+  };
+  var blob = new Blob();
+  reader.readAsText(blob);
+  // We've started reading 5 blobs in this iteration of the event-loop.
+
+  // Do this in the current iteration of the event-loop.
+  performance.mark("start");
+});