Bug 1689343 - Enlarge the iframe viewport in bug1448730.html. r=spohl

Without this the non-native theme fails the last assertion of the test.

I couldn't reproduce locally but the word that it's supposed to be
selected is very close to being out of the iframe viewport on my
machine, so it'd make sense if the new non-native theme, which has a bit
more padding, causes the word to move a bit further down.

I've confirmed this fixes the issue.

Differential Revision: https://phabricator.services.mozilla.com/D103317

layout/base/tests/test_bug1448730.html

@@ -21,6 +21,8 @@ SimpleTest.waitForExplicitFinish();
 SpecialPowers.pushPrefEnv({"set": [['layout.accessiblecaret.enabled', true]]}, () => {
   let iframe = document.createElement("iframe");
   iframe.src = "bug1448730.html";
+  iframe.style.width = "100%";
+  iframe.style.height = "100%";
   document.getElementById('container').appendChild(iframe);
 });
 </script>