Auto merge of #5753 - JIoJIaJIu:timeline, r=jdm

RequestAnimationFrame
[Task](https://github.com/servo/servo/issues/5681)

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tests/html/test_request_animation_frame.html

@@ -0,0 +1,5 @@
+<script>
+window.requestAnimationFrame(function (time) {
+    alert("time " + time);
+});
+</script>

tests/wpt/include.ini

@@ -1,6 +1,8 @@
 skip: true
 [2dcontext]
   skip: false
+[animation-timing]
+  skip: false
 [dom]
   skip: false
 [domparsing]

tests/wpt/metadata/animation-timing/callback-invoked.html.ini

@@ -0,0 +1,4 @@
+[callback-invoked.html]
+  type: testharness
+  [requestAnimationFrame callback is invoked at least once before the timeout]
+    expected: FAIL