Summary:
Changelog: [Internal]
# Problem
`MountingCoordinator` holds a pointer to instance of `MountingOverrideDelegate` which becomes invalid.
# Solution
Use `std::weak_ptr` instead of raw pointer so it is possible to tell whether the pointer is expired.
Reviewed By: JoshuaGross
Differential Revision: D21905351
fbshipit-source-id: c7bf9635742a6ec086a03ba83202e46e1f1f373f