Auto merge of #17812 - brainlessdeveloper:use-resolved-url-instead-of-original, r=emilio

Introduce ComputedUrl

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

Use the new `ComputedUrl` type for computed types and `SpecifiedUrl` for specified types instead of using the `SpecifiedUrl` implementation for both.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #17625 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because this is an implementation change and tests already exist.

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -13536,6 +13536,12 @@
      {}
     ]
    ],
+   "css/get-computed-style-for-url.html": [
+    [
+     "/_mozilla/css/get-computed-style-for-url.html",
+     {}
+    ]
+   ],
    "css/import_serialization.html": [
     [
      "/_mozilla/css/import_serialization.html",
@@ -23357,6 +23363,10 @@
    "13a3f8dd23fa28c0b2ad2fe0662d29a27a569e74",
    "support"
   ],
+  "css/get-computed-style-for-url.html": [
+   "2e90c0abd6c83bb11113f39a557a4c1c1c24364b",
+   "testharness"
+  ],
   "css/green.png": [
    "15e39f6df8def787cefcfb30e27de5f43da65c9a",
    "support"