chore(perm interface): Fix return value handling

[jugglinmike]

Unlike the other methods defined in this section, revoke is defined as
a series of steps to be run in parallel. Returning a value is
meaningless in this context. Instead, the result of the operation should
be communicated via the Promise instance that is defined and
synchronously returned prior to the parallelized steps.

[jyasskin]

Sorry it took me so long to look at this. The "real" problem here is that revoke shouldn't run all of its steps in parallel. We're specifically not allowed to use "converted to an IDL value" from parallel steps, since ECMAScript objects are single-threaded.

It should go parallel before step 3, and then have step 5 call into the middle of query() with the already-converted typedDescriptor.

I'm finally working on pulling request() and revoke() into an incubation, and I'll fix this there.