fix(react-dom): access iframe contentWindow instead of contentDocument

MDN has a list of methods for obtaining the window reference of an
iframe:

https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage#Syntax

packages/react-dom/src/client/ReactInputSelection.js

@@ -44,10 +44,10 @@ function getActiveElementDeep() {
   let win = window;
   let element = getActiveElement();
   while (element instanceof win.HTMLIFrameElement) {
-    // Accessing the contentDocument of a HTMLIframeElement can cause the browser
+    // Accessing the contentWindow of a HTMLIframeElement can cause the browser
     // to throw, e.g. if it has a cross-origin src attribute
     try {
-      win = element.contentDocument.defaultView;
+      win = element.contentWindow;
     } catch (e) {
       return element;
     }