This is the first step in making Flutter aware of and responsive to Z
bound overflow. On its own this patch shouldn't result in any changes in
behavior. This will need to be followed up with a patch in Fuchsia's
Flutter runner to set the Z bounds after this lands, and another patch
in the engine actually implementing the desired overflow behavior.

This Z bound info is routed through the engine itself to make sure the
bounds in flow are truly consistent from the Fuchsia runner. However
these values should eventually be exposed to the framework as well.