IOS: Screenreader (VoiceOver) doesn't recognize anything to read

Hi,
the IOS Screenreader (VoiceOver) does not recognize any content when you show some website using the `WebviewScaffold` widget.
On android (using TalkBack) it works fine.

Here are my system informations and the flutter doctor output.

I am happy about any help.
Best regards

## System info

Issue occurs on: iOS
Plugin version: 0.3.11
Flutter doctor -v output:

```
flutter doctor -v
[✓] Flutter (Channel stable, 2.0.4, on macOS 11.2.3 20D91 darwin-x64, locale en-DE)
• Flutter version 2.0.4 at /Users/fabian/Documents/flutter
• Framework revision b1395592de (5 weeks ago), 2021-04-01 14:25:01 -0700
• Engine revision 2dce47073a
• Dart version 2.12.2

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
• Android SDK at /Users/fabian/Library/Android/sdk
• Platform android-30, build-tools 29.0.2
• ANDROID_HOME = /Users/fabian/Library/Android/sdk
• Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 12.4, Build version 12D4e
• CocoaPods version 1.10.1

[✓] Chrome - develop for the web
• Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 4.0)
• Android Studio at /Applications/Android Studio.app/Contents
• Flutter plugin version 51.0.1
• Dart plugin version 193.7547
• Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)

[✓] VS Code (version 1.55.2)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 3.20.0

[✓] Connected device (3 available)
• Moto G 5 (mobile) • ZY32232ZCX • android-arm • Android 8.1.0 (API 27)
• iPhone (mobile) • 00008030-0011610E3AD9802E • ios • iOS 14.4.2
• Chrome (web) • chrome • web-javascript • Google Chrome 90.0.4430.93
```

まず、WebviewScaffoldウィジェットを使用してVoiceOverでiOS上の問題を再現し、その後、Android TalkBackでの動作と比較します。VoiceOverが表示されたウェブサイトのコンテンツを認識して読み上げられるようになれば、問題は解決です。報告では、ソースファイルもテストも特定されていません。

索引モデルが issue の本文から書いたものです。