Inter-process StructuredClone of SharedArrayBuffer

[asajeffrey]

In https://searchfox.org/mozilla-central/rev/ca869724246f4230b272ed1c8b9944596e80d920/js/public/StructuredClone.h#218-224, the policy for inter-process cloning is:

    // In the JS engine, SharedArrayBuffers can only be cloned intra-process
    // because the shared memory areas are allocated in process-private memory.
    // Clients should therefore deny SharedArrayBuffers when cloning data that
    // are to be transmitted inter-process.
    //
    // Clients should also deny SharedArrayBuffers when cloning data that are to
    // be transmitted intra-process if policy needs dictate such denial.

We need to think about what this means for Servo: can we use the type system to distinguish beween inter- vs intra-process structured clone data?