Summary:
Before this change, `mountingOverrideDelegate` was proxied via `Scheduler::startSurface` and `ShadowTree::ShadowTree` constructor down to `MountingCordinator`. Now we set it on the `MountingCoordinator` directly.
Changelog: [Internal] Fabric-specific internal change.
Reviewed By: sammy-SC
Differential Revision: D26049076
fbshipit-source-id: 7f1ecf2c8b6f264a7e59d19881464fe529c53d30