Message replies are sometimes mixed up

When we send a message to the injected iframe, we assume the first reply we see is the response to that message. That might not be the case if we call the same method multiple times in quick succession.

To ensure we don't get our replies mixed up, we should attach a unique identifier to each message and look for it in the reply.

Inspect the code that sends messages to the injected iframe and receives replies. Trace how concurrent calls are currently matched, then add per-message identifiers and verify that each reply is associated with the request that created it.

Written by the indexing model from the issue text.