[push-api] Remove [NoInterfaceObject] from PushMessageData (#123)

Please stop using [NoInterfaceObject]. It's bad for extensibility. For instance, if we I want to polyfill a new processing method on `PushMessageData` (i.e., .fancyMsgFormat()), I won't be able to do that without it being a huge pain the ass.



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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/123