Auto merge of #24677 - jdm:iframe-sizing-cleanup, r=asajeffrey

Ensure all iframes are sized correctly at creation

Following in the path of #22395, these commits serve several purposes:
* prevent layout instability early during iframe loads caused by a succession of resize events
* reduce the complexity of determining what actual DPI and initial window size values are being used at startup
* ensure that all documents have a correct initial viewport size at creation

These changes fix problems that were exposed by the changes in #24462 but are independent of that PR.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes

tests/wpt/mozilla/meta/MANIFEST.json

@@ -19034,7 +19034,7 @@
    "testharness"
   ],
   "mozilla/mql_borrow.html": [
-   "17ee0dc48a30933429cb901760ef1b074ed56b6e",
+   "2f738c738f2efc7b36fcdab31741a34037e673f7",
    "testharness"
   ],
   "mozilla/multiple_redirects.html": [

tests/wpt/mozilla/tests/mozilla/mql_borrow.html

@@ -23,6 +23,7 @@
         frame.height=600;
         frame.width=400;
     }));
+    frame.width = 601;
 
     </script>