Since JSWindowActors don't have direct access to synchronous messaging,
ChromeScript callers are going to need to migrate to asynchronous messaging
and queries instead.
Since there's no comparable API to sendQuery for frame message managers, this
patch adds a stub that uses synchronous messaging, but makes the API appear
asynchronous, and migrates callers to use it instead of direct synchronous
messaging. This will be replaced with a true synchronous API in the actor
migration.
Fortunately, most of the time, this actually leads to simpler code. The
`sendQuery` API doesn't have the odd return value semantics of
`sendSyncMessage`, and can usually just be used as a drop-in replacement. Many
of the `sendSyncMessage` callers don't actually use the result, and can just
be changed to `sendAsyncMessage`. And many of the existing async messaging
users can be changed to just use `sendQuery` rather than sending messages and
adding response listeners.
However, the APZ code is an exception. It relies on intricate properties of
the event loop, and doesn't have an easy way to slot in promise handlers, so I
migrated it to using sync messaging via process message managers instead.
Differential Revision: https://phabricator.services.mozilla.com/D35055
1eb6dde4ac