cdc18f9d61
Adds a `destruction_callback` member to `FlutterTaskRunnerDescription` that provides embedder developers a means of performing any cleanup of resources tied to the lifetime of the task runner. In the case of the macOS embedder, the task runner holds a reference to the engine itself that is used in the `post_task_callback` and whose lifetime needs to match/exceed that of the task runner. This refactoring allows us to centralise all related retain count manipulation around task runner setup. This is a refactor of the lifecycle bits of https://github.com/flutter/engine/pull/13300. [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style