Auto merge of #27032 - mrobinson:fractional-iteration, r=jdm

animations: Finish support for fractional iteration counts

This change also improves support for creating animations with negative
delays, as that is necessary to test support for fractional iteration
lengths.

Fixes: #14858

<!-- 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: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #14858
- [x] There are tests for these changes

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -12871,7 +12871,7 @@
    "css": {
     "animations": {
      "animation-delay.html": [
-      "0d2053a9134d8ff0ade7b5dc37ecfce305557c44",
+      "f54cf2b9bca93e82b177243b9d754e4ae3bf15fa",
       [
        null,
        {}
@@ -12887,7 +12887,7 @@
       ]
      ],
      "animation-fill-mode.html": [
-      "9602ec9f0e0eb1f6efcc2e7bd95181ef65339bae",
+      "ac3062879af9836768890d653f4b29b6165b6a45",
       [
        null,
        {}