Auto merge of #20012 - alexfjw:avoid_Window-GetComputedStyle_when_checking_for_display-none, r=emilio

Avoid `Window::GetComputedStyle` when checking for `display: none`

<!-- Please describe your changes on the following line: -->
Refactored Window::GetComputedStyle to use Element::Style.

Not sure which tests are relevant, but I've ran the dom, fetch & 2dcontext wpt tests. They don't seem to give errors.

---
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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 #19885.

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- [x] These changes do not require tests because it's a refactoring task

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tests/wpt/mozilla/meta/MANIFEST.json

@@ -70817,7 +70817,7 @@
    "testharness"
   ],
   "mozilla/transitionend_safety.html": [
-   "778e43b049aa421bad7f86eb03d0955576a84ce0",
+   "16d238e94b2cc2843c9aee4210db43d496bb3114",
    "testharness"
   ],
   "mozilla/union.html": [

tests/wpt/mozilla/tests/mozilla/transitionend_safety.html

@@ -11,7 +11,6 @@
     elem.textContent = 'hi there';
     elem.style.transition = 'color 10ms';
     elem.style.color = 'black';
-    elem.ontransitionend = t.step_func_done();
 
     t.step_timeout(function() {
       elem.style.color = 'red';
@@ -20,6 +19,7 @@
         document.body.removeChild(elem);
         elem = null;
         window.gc();
+        t.step_timeout(t.step_func_done(), 100);
       }, 0);
     }, 0);
   }, 'Nodes cannot be GCed while a CSS transition is in effect.');