Skip http/tests/site-isolation/double-iframe.html on iOS

https://bugs.webkit.org/show_bug.cgi?id=264422

Unreviewed.

#20120 added a new test that fails on iOS
because of some unrelated drawing issues that also affect other site-isolation
tests.  Skip this new test like we do the others until the underlying issue is
fixed.

* LayoutTests/platform/ios/TestExpectations:

Canonical link: https://commits.webkit.org/270398@main

LayoutTests/platform/ios/TestExpectations

@@ -4568,6 +4568,7 @@ imported/w3c/web-platform-tests/css/css-lists/content-property/marker-text-match
 
 http/tests/site-isolation/mouse-events/ [ Skip ]
 webkit.org/b/257904 http/tests/site-isolation/basic-iframe.html [ Skip ]
+webkit.org/b/257904 http/tests/site-isolation/double-iframe.html [ Skip ]
 webkit.org/b/257904 http/tests/site-isolation/draw-after-navigation.html [ Skip ]
 webkit.org/b/257904 http/tests/site-isolation/selection-focus.html [ Skip ]