Delay between removal of bottom navigation bar and resizing of the webview

## System info

Issue occurs on: both
Plugin version: 0.3.8
Flutter doctor output: see below

## Steps to Reproduce

1. Use the `WebviewScaffold` with a conditionally shown `bottomNavigationBar`, e.g. like this:
```
@override
Widget build(BuildContext context) {
return WebviewScaffold(
url: _baseUrl,
bottomNavigationBar: _shouldShowBottomBar ? _buildBottomAppBar() : null,
);
}
```
2. Change the state of the `_shouldShowBottomBar` variable during runtime, e.g. in a `onScrollYChanged` handler like this:
```
void _onScrollYChanged(double offset) {
setState(() {
_shouldShowBottomBar = offset < 50;
});
}
```
3. Test the hiding/showing the of bottom navigation bar.
4. Notice that when the bottom navigation bar is hidden, the space that gets available is not immediately taken by the web view. It takes some time, during which the area is white. This looks bad.
Example:


## Logs

`flutter run --verbose`:
There are many log lines created like the following when the bottom nav bar is hidden, but nothing that is actually relevant is logged.
```
[ +16 ms] D/eglCodecCommon( 7996): setVertexArrayObject: set vao to 1 (1) 1 6
```

`flutter analyze`:
```
No issues found! (ran in 3.2s)
```

`flutter doctor -v`:
```
[√] Flutter (Channel stable, v1.9.1+hotfix.2, on Microsoft Windows [Version 10.0.17763.737], locale de-AT)
• Flutter version 1.9.1+hotfix.2 at C:\Android\flutter
• Framework revision 2d2a1ffec9 (12 days ago), 2019-09-06 18:39:49 -0700
• Engine revision b863200c37
• Dart version 2.5.0

[√] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
• Android SDK at C:\Android\Sdk
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-29, build-tools 29.0.2
• ANDROID_SDK_ROOT = C:\Android\Sdk
• Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
• Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b03)
• All Android licenses accepted.

[√] Android Studio (version 3.5)
• Android Studio at C:\Program Files\Android\Android Studio
• Flutter plugin version 39.0.3
• Dart plugin version 191.8423
• Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b03)

[√] Connected device (1 available)
• Android SDK built for x86 • emulator-5554 • android-x86 • Android 10 (API 29) (emulator)

• No issues found!
```