Bug 1063247: Allow implicit construction of JS::ubi::Node from JS::HandleValue. r=terrence

This seems reasonable, since a ubi::Node is really just a pointer with some
introspection support, so the value and the Node are pretty similar. And I don't
see cases where you'd want an error.
Bug 1063247: Amend JS::ubi::Node::size and its implementations to expect a mozilla::MallocSizeOf function. r=terrence

Note that JS::ubi::Node::size has no callers at present, so we can change its
type without changing any callers.
Bug 1063247: Let embeddings tell Debugger how to find the size of a malloc'd block of memory. r=terrence