Add polling to guarantee chromecast initialization

Sometimes the window['__onGCastApiAvailable'] callback is fired before castaway has a chance to initialize.

The chrome sender docs suggest having a timeout to address the loading of the API.
https://developers.google.com/cast/docs/chrome_sender

Polling for initialization guarantees that the initialization occurs.
Can we switch these two lines? If there is an error in `initializeCastApi` (which includes executing a consumer-provided callback), it could cause this interval to keep going.