Summary:
Previously we computed the list of nodes that need to be notified about layout changes using a list of mutation instructions. That was fine, but that's not really compatible with some other changes that I plan to make, so I decided to change it (make it better).
Besides the better design (debatable; fewer dependencies to unrelated moving pieces), here is why I believe the new way is more performant:
* The new approach has no `dynamic_casts`, whereas the previous has tons of them (two per a mutation). If a `dynamic_cast` takes 10 ns, for 500 nodes it can take up to 5ms only for casts. (Non-scientific assumption.)
* After removing dependency to mutation instruction, we can enable flattening for views which have `onLayout` event.
Reviewed By: mdvacca
Differential Revision: D15110725
fbshipit-source-id: 31a657ccfd02441734ad1d71a833653223163289
ea8a57116f