Don't use a separate channel to communicate window size to the
constellation.

This will make sandboxing much easier since we won't need a
separate Unix pipe just for this.
Remove WrapNativeParent and nearby cleanup.
Remove Reflectable::GetParentObject.
auto merge of #1198 : Ms2ger/servo/GetParentObject, r=jdm
auto merge of #1182 : pcwalton/servo/no-size-channel, r=metajack

This will make sandboxing much easier since we won't need a
separate Unix pipe just for this.
Build some dependencies with rustpkg

In collaboration with Jack Moffitt.