Progress Indicator

Web pages appear if the widget in the `initialChild` is wrapped in `Center()` widget

```dart
WebviewScaffold(
url: _url,
hidden: true,
initialChild: Center(
child: LinearProgressIndicator()
)
)
```

but does not appear if it is not wrapped in a `Center() `widget

```dart
WebviewScaffold(
url: _url,
hidden: true,
initialChild: LinearProgressIndicator()
)
```

Reproduce the two WebviewScaffold examples using initialChild with and without Center and compare their visible behavior. Trace how WebviewScaffold handles initialChild during page loading, then verify that a LinearProgressIndicator is displayed in the unwrapped case as well.

Written by the indexing model from the issue text.