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

facebook#15099)

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

fix(react-dom): check if iframe belongs to the same origin

Accessing the contentDocument of a HTMLIframeElement can cause the browser
to throw, e.g. if it has a cross-origin src attribute.
Safari will show an error in the console when the access results in "Blocked a frame with origin". e.g:

```javascript
try {
 $0.contentDocument.defaultView
} catch (err) {
  console.log('err', err)
}

> Blocked a frame with origin X from accessing a frame with origin Y. Protocols, domains, and ports must match.
> err – TypeError: null is not an object (evaluating '$0.contentDocument.defaultView')
```

A safety way is to access one of the cross origin properties: Window or Location
Which might result in "SecurityError" DOM Exception and it is compatible to Safari.

```javascript
try {
 $0.contentWindow.location.href
} catch (err) {
 console.log('err', err)
}

> err – SecurityError: Blocked a frame with origin "http://localhost:3001" from accessing a cross-origin frame. Protocols, domains, and ports must match.
```

https://html.spec.whatwg.org/multipage/browsers.html#integration-with-idl

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

@@ -40,15 +40,29 @@ function isInDocument(node) {
   );
 }
 
+function isSameOriginFrame(iframe) {
+  try {
+    // Accessing the contentDocument of a HTMLIframeElement can cause the browser
+    // to throw, e.g. if it has a cross-origin src attribute.
+    // Safari will show an error in the console when the access results in "Blocked a frame with origin". e.g:
+    // iframe.contentDocument.defaultView;
+    // A safety way is to access one of the cross origin properties: Window or Location
+    // Which might result in "SecurityError" DOM Exception and it is compatible to Safari.
+    // https://html.spec.whatwg.org/multipage/browsers.html#integration-with-idl
+
+    return typeof iframe.contentWindow.location.href === 'string';
+  } catch (err) {
+    return false;
+  }
+}
+
 function getActiveElementDeep() {
   let win = window;
   let element = getActiveElement();
   while (element instanceof win.HTMLIFrameElement) {
-    // Accessing the contentDocument of a HTMLIframeElement can cause the browser
-    // to throw, e.g. if it has a cross-origin src attribute
-    try {
-      win = element.contentDocument.defaultView;
-    } catch (e) {
+    if (isSameOriginFrame(element)) {
+      win = element.contentWindow;
+    } else {
       return element;
     }
     element = getActiveElement(win.document);