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
```

Comienza reproduciendo el problema en iOS con VoiceOver usando el widget WebviewScaffold y, después, compara el comportamiento con Android TalkBack. El problema se resuelve cuando VoiceOver reconoce y puede leer el contenido del sitio web mostrado; en el informe no se identifica ningún archivo fuente ni ninguna prueba.

Escrito por el modelo de indexación a partir del texto del issue.