Figure out if TrustedXYZ objects should be Transferable

[koto]

Transferable interface allows an object to be transferred between different execution contexts. Allowing TT to be Transferable would make it possible to e.g. create objects in a worker and consume them in a main document. However, this might also make it possible to send them over postMessage channels. Obviously, this is undesirable, unless we track which policies created those objects (policies are per realm).

Initially, I think TT should NOT be Transferable, but leaving this open in case some use cases arise.

[koto]

I've made a decision. Types should not be Transferable for sure. Apart from aforementioned reasons, it makes it tricky to push the TT under Origin Trials, (one document has the trial token, but the other might not).