Detect if Webview scroll reach the end

I'm implementing the EULA WebView with the "Accept" button which must be visible only when the user scrolls to the bottom of the web page.
The current scroll position is already implemented and it's working great:
```
webviewPlugin
..onScrollYChanged.listen((double offsetY) {
}
```
**Is it possible to get the max scroll value?**
(getMaxScrollAmount or similar...)

Start from the webviewPlugin.onScrollYChanged listener described in the issue and inspect how the plugin exposes scroll offsets. Determine how maximum scroll information could be exposed so callers can detect the page bottom for the EULA Accept button, then verify the behavior with the plugin's existing WebView flow.

索引モデルが issue の本文から書いたものです。