Keyboard not showing up in iOS

## System info

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

```
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 1.22.4, on macOS 11.0.1 20B29 darwin-x64, locale en-IN)

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
[✓] Xcode - develop for iOS and macOS (Xcode 12.2)
[!] Android Studio (version 4.1)
✗ Flutter plugin not installed; this adds Flutter specific functionality.
✗ Dart plugin not installed; this adds Dart specific functionality.

[✓] Connected device (1 available)

! Doctor found issues in 1 category.
```

## Steps to Reproduce

Please see my [code here](https://gist.github.com/ash007osh/4694a290edb5a6603b65c655c4da5002)

## Logs

```
Nothing related to this particular page
```

Unrelated Issues:
```
info • 'title' is deprecated and shouldn't be used. Use "label" instead, as it allows for an improved text-scaling
experience. This feature was deprecated after v1.19.0. • lib/navbar.dart:11:13 • deprecated_member_use
info • 'title' is deprecated and shouldn't be used. Use "label" instead, as it allows for an improved text-scaling
experience. This feature was deprecated after v1.19.0. • lib/navbar.dart:15:13 • deprecated_member_use
info • 'title' is deprecated and shouldn't be used. Use "label" instead, as it allows for an improved text-scaling
experience. This feature was deprecated after v1.19.0. • lib/navbar.dart:19:13 • deprecated_member_use

3 issues found. (ran in 116.9s)
```

```
[✓] Flutter (Channel stable, 1.22.4, on macOS 11.0.1 20B29 darwin-x64, locale en-IN)
• Flutter version 1.22.4 at /Users/innovationswork/Desktop/flutter
• Framework revision 1aafb3a8b9 (9 days ago), 2020-11-13 09:59:28 -0800
• Engine revision 2c956a31c0
• Dart version 2.10.4

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
• Android SDK at /Users/innovationswork/Library/Android/sdk
• Platform android-29, build-tools 29.0.3
• 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-b3-6915495)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 12.2)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 12.2, Build version 12B45b
• CocoaPods version 1.9.3

[!] Android Studio (version 4.1)
• Android Studio at /Applications/Android Studio.app/Contents
✗ Flutter plugin not installed; this adds Flutter specific functionality.
✗ Dart plugin not installed; this adds Dart specific functionality.
• Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)

[✓] Connected device (1 available)
• iPhone SE (2nd generation) (mobile) • 0D3AA587-7F8B-475F-8C6F-5F8EE5F505EA • ios •
com.apple.CoreSimulator.SimRuntime.iOS-14-2 (simulator)

! Doctor found issues in 1 category.```

まず、flutter_webview_plugin 0.3.11、Flutter 1.22.4、iOS 14.2 を使用して、リンクされた gist を iOS で再現し、その後プラグインの iOS キーボードと WebView の動作を調査します。WebView 内の入力欄がフォーカスを受け取ったときにソフトウェアキーボードが表示されれば完了です。issue では、リポジトリ内のファイルやテストは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。