Property supportMultipleWindows of Android WebView needs to have onCreateWindow implemented

Issue occurs on: Android
Plugin version: 0.3.8
Flutter doctor output:

```
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel beta, v1.9.1+hotfix.2, on Mac OS X 10.14.6 18G87, locale en-RO)

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
[✓] Xcode - develop for iOS and macOS (Xcode 10.2.1)
[✓] Android Studio (version 3.5)
[✓] VS Code (version 1.38.1)
[✓] Connected device (2 available)

• No issues found!
```

I am trying to implement login with Facebook. During the process, Facebook needs to open a new window and then to do the redirect. On Android native you would set supportMultipleWindows to true on the WebView and you would catch the onCreateWindow() callback and open the url in a new WebView.

I tried setting supportMultipleWindows: true on the WebViewScaffold but then pressing the Login Facebook button does not do anything.

The documentation on Android WebView says that if you set supportMultipleWindows: true you NEED to override onCreateWindow(). Is this possible with your plugin? Am I missing something obvious? Why would you have the supportMultipleWindows property on the WebViewScaffold but not support for the corresponding onCreateWindow()?

Any help is greatly appreciated because I am stuck.

ファイル、テスト、エントリポイントは指定されていません。まず、Android WebViewScaffold による supportMultipleWindows と onCreateWindow callback の処理を追跡します。Facebook のログインフローが新しいウィンドウを開き、Android 上でリダイレクトを完了すれば完了です。

索引モデルが issue の本文から書いたものです。