Message data to send to the server. Case-insensitive goog. and google,
as well as case-sensitive collapse_key are disallowed as key prefixes.
Sum of all key/value pairs should not exceed gcm.MAX_MESSAGE_SIZE.
The ID of the message. It must be unique for each message in scope of the
applications. See the Cloud Messaging
documentation
for advice for picking and handling an ID.
Time-to-live of the message in seconds. If it is not possible to send the
message within that time, an onSendError event will be raised. A
time-to-live of 0 indicates that the message should be sent immediately or
fail if it's not possible. The default value of time-to-live is 86,400
seconds (1 day) and the maximum value is 2,419,200 seconds (28 days).