Auto merge of #22395 - jdm:initial-iframe-size, r=Manishearth

Initialize iframe viewport immediately

This is probably a significant cause of unstable test results, and it finally bothered me enough to fix it. The solution isn't great for performance (a sync layout query every time a iframe is added to a document), but performance needs to follow correctness.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #14364 and fix #15689 and fix #15688.
- [x] There are tests for these changes

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tests/wpt/metadata/css/cssom-view/matchMedia.xht.ini

@@ -4,18 +4,9 @@
   [window.matchMedia exists]
     expected: FAIL
 
-  [MediaQueryList.matches for "(max-width: 199px), all and (min-width: 200px)"]
-    expected: FAIL
-
   [MediaQueryList.matches for "(min-aspect-ratio: 1/1)"]
     expected: FAIL
 
-  [MediaQueryList.matches for "(width: 200px)"]
-    expected: FAIL
-
-  [MediaQueryList.matches for "(min-width: 150px)"]
-    expected: FAIL
-
   [Resize iframe from 200x100 to 200x50, then to 100x50]
     expected: NOTRUN
 

tests/wpt/metadata/css/cssom/getComputedStyle-detached-subtree.html.ini

@@ -1,2 +1,8 @@
 [getComputedStyle-detached-subtree.html]
   expected: ERROR
+  [getComputedStyle returns no style for element in non-rendered iframe (display: none) from iframe's window]
+    expected: FAIL
+
+  [getComputedStyle returns no style for element in non-rendered iframe (display: none)]
+    expected: FAIL
+