Allow saving child surfaces.

We can and should save child surfaces as the client side window
state hasn't been torn down in the case where we would otherwise save.
So, if we destroy the Surface, the client will continue to render
and crash.

Bug: 26793431
Bug: 25780116
Bug: 26777815
Change-Id: Ia043b7af24b9370fc17a5b57226566f94c08ba4e