Async unload operation for an AssetBundle.
| WaitForCompletion | Synchronously waits for the operation to complete. |
| allowSceneActivation | Allow Scenes to be activated as soon as it is ready. |
| isDone | 動作が終了したか確認します（読み取り専用） |
| priority | 非同期で読み込む際の優先順位を設定します。 |
| progress | 進捗状況を表示します（読み取り専用） |
| completed | Event that is invoked upon operation completion. An event handler that is registered in the same frame as the call that creates it will be invoked next frame, even if the operation is able to complete synchronously. If a handler is registered after the operation has completed and has already invoked the complete event, the handler will be called synchronously. |