Summary:
See the comment in RCTMountingTransactionObserving first.
I think we have to add this to the iOS mounting layer to be able reasonably easy implement things like:
* MovableNavigationBar: seems, currently we don't handle the situation when the container was mounted first and the nested scroll view second.
* TTI component: It does not have access to telemetry.
The protocol is meant to replace `RCTSurfacePresenterObserver`.
Changelog: [Internal] Fabric-specific change.
Reviewed By: mdvacca
Differential Revision: D16270107
fbshipit-source-id: 2d4bdb7d0092cc214cc433fc633e41e58f6677df
