Summary:
@public
Now it's clear that we don't need to store/handle ShadowTree objects as `shared_ptr`s; Scheduler should own it. This diff changes that to using unique_ptr and removes a base class of ShadowTree.
Reviewed By: mdvacca
Differential Revision: D9403567
fbshipit-source-id: 6e411714b632a04233fd5b25c8ab7cdd260105fd