veyron2/ipc: 3 of n. 'Invoker and Signature' rationalisation.
veyron2/ipc: 3 of n. 'Invoker and Signature' rationalisation.

See https://veyron-review.googlesource.com/#/c/6278/

Change-Id: I86699bc63ea556f1b6959981dfc3212b17a75b50

vanadium/playground,vanadium/playground,vanadium/playground,vanadium/playground