WebView is not fullscreen

Hi there, i want just confirm is this a bug or not?
Maybe I'm doing something wrong? Or maybe this is a Xiaomi specific bug when renders webview widget.

red offset on the right and bottom.

[Screenschot](https://i.imgur.com/Ahab0yB.jpg)

in the UI is nothing special:
```
return MaterialApp(
home: Scaffold(
body: SafeArea(
child: WebviewScaffold(
url: initialUrl,
initialChild: Container(
color: Colors.red,
child: Center(
child: Text("Loading..."),
),
),
),
bottom: true,
left: true,
right: true,
top: true,
),
),
);
```

Thank you for help in advance =)

首先，在报告的 Xiaomi 设备上使用 WebviewScaffold 重现示例，并将其布局与屏幕截图进行比较。检查红色偏移是否来自 SafeArea、scaffold 的 bottom/left/right/top 设置，或插件的 WebView 渲染。完成的标准是确定这是插件 bug 还是应用程序布局问题。

由索引模型根据 Issue 内容生成。