Calling dispose() and close() within onUrlChanged does not pop the WebviewScaffold

When trying to pop the WebviewScaffold by calling `FlutterWebViewPlugin.dispose()` and `FlutterWebViewPlugin.close()` after having been redirected, instead of popping the Route, the `_WebviewPlaceholder` only switches to the `child` Widget (`initalChild` or `CircularProgressIndicator`), rather than returning to the previous Route.

Start by tracing WebviewScaffold and its _WebviewPlaceholder around onUrlChanged, then follow how FlutterWebViewPlugin.dispose() and close() handle a redirected route. Reproduce the redirect case and verify that disposing or closing pops the WebviewScaffold to the previous Route instead of showing the child Widget.

Written by the indexing model from the issue text.