Weird 1 Pixel offset of the WebView over the initialChild

I have such a problem you can see on the right of the screenshot - the initialChild which is just red colored Container can bee seen there.

I'm using Flutter 1.19.0-0.0pre.

Reproduction code
```dart
WebviewScaffold(
url: 'https://twitch.com',
initialChild: Container(
color: Colors.red,
),
)
```


Start by reproducing the 1-pixel offset with WebviewScaffold, the supplied Twitch URL, and the red initialChild on the reported Flutter version. Trace how WebviewScaffold lays out the initialChild and WebView; done means the red initialChild is no longer visible beside the WebView.

Written by the indexing model from the issue text.