cleanCookies does not work on iOS

## System info

Issue occurs on: iOS
Plugin version: 0.3.10+1
Flutter doctor output:

```
[✓] Flutter (Channel stable, v1.12.13+hotfix.5, on Mac OS X 10.15.2 19C57, locale en-UA)

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
[✓] Xcode - develop for iOS and macOS (Xcode 11.2.1)
[✓] Android Studio (version 3.5)
[✓] Connected device (3 available)

```

## Steps to Reproduce

1. Use WebviewScaffold's clearCookies property or FlutterWebviewPlugin.cleanCookies method.

Currently it's not possible to drop cookies when webview is not on the screen using iOS device.
There is a check in FlutterWebviewPlugin.m#cleanCookies - if (self.webview != nil) despite it does not depend on current webview instance.
Also there is no result when calling FlutterWebviewPlugin.cleanCookies if self.webview is null or iOS version < 9.

Are there any reasons it checks if the webview is instantiated? It works as expected on Android and should have same behaviour on different platforms.

Thank you!

从 FlutterWebviewPlugin.m 中的 cleanCookies 开始，检查 self.webview 守卫以及 iOS 9 以下版本的行为。验证在屏幕上没有 Webview 时，WebviewScaffold 的 clearCookies 属性和 FlutterWebviewPlugin.cleanCookies 调用是否正常工作，并确认该调用能够与 Android 一致地提供结果。

由索引模型根据 Issue 内容生成。