Summary:
Setting `windowSize = 0` doesn't make sense. Let's make sure we catch this problem in the constructor so that it doesn't cause inexplicable list behavior.
Also fixed an invariant in `VirtualizeUtils` that is meant to prohibit non-monotonically-increasing offset arrays. As written, the invariant condition can never actually be violated.
Reviewed By: sahrens
Differential Revision: D6625302
fbshipit-source-id: b2a983cbe7bb5fbe0aed7c5d59e69a8a00672993