Fix custom element registry define test

The test relies on DOM elements with ids beind added
as properties of the window. Servo does not do this, and
this is not a best practice.

tests/wpt/metadata/MANIFEST.json

@@ -554144,7 +554144,7 @@
    "testharness"
   ],
   "custom-elements/custom-element-registry/define.html": [
-   "724060b276a1f3254fd226e329631005ec5a5e87",
+   "4f3d4da1aae8efab035a4b951147904d71e7de3b",
    "testharness"
   ],
   "custom-elements/disconnected-callbacks.html": [

tests/wpt/web-platform-tests/custom-elements/custom-element-registry/define.html

@@ -12,6 +12,7 @@
   // https://html.spec.whatwg.org/multipage/scripting.html#element-definition
 
   // Use window from iframe to isolate the test.
+  const iframe = document.getElementById("iframe");
   const testWindow = iframe.contentDocument.defaultView;
   const customElements = testWindow.customElements;