a01206ad39
In the future, `FlutterEngineAddView` will be added to the embedder API to allow embedders to add views. `FlutterEngineAddView` will accept a callback that notifies the embedder once the view has been added. This embedder API will be powered by `Shell::AddView`. This change adds a completion callback to `Shell::AddView` to prepare for the embedder API. Design doc: https://flutter.dev/go/multi-view-embedder-apis Part of https://github.com/flutter/flutter/issues/144806 Part of https://github.com/flutter/flutter/issues/142845 [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style