Registers the application with FCM. The registration ID will be returned
by the callback. If register is called again with the same list of
senderIds, the same registration ID will be returned.
senderIds A list of server IDs that are allowed to send messages to
the application. It should contain at least one and no more than 100
sender IDs.
returns Function called when registration completes. It should check
runtime.lastError for error when registrationId is empty.
Sends a message according to its contents.
message A message to send to the other party via FCM.
returns A function called after the message is successfully queued for
sending. runtime.lastError should be checked, to ensure a message was
sent without problems.
Unregisters the application from FCM.
returns A function called after the unregistration completes.
Unregistration was successful if runtime.lastError is not set.