When we migrate SpecialPowers to a JSWindowActor, it will no longer be able to
use synchronous IPC messaging, which means that its current synchronous APIs
will have to become asynchronous.
This patch doesn't change the behavior of those functions, but it does change
their callers to `await` their return values rather than using them directly.
This pattern will work the same whether the functions return a promise or a
plain value, which simplifies the migration.
Differential Revision: https://phabricator.services.mozilla.com/D35053
4753549aee