setContentMetadata(metadata) {
    this.metadata_ = metadata;
  }
Set all Cast content metadata, as defined by the Cast SDK.
Should be called from an appDataCallback.

For a simpler way to set basic metadata, see:
 - setContentTitle()
 - setContentImage()
 - setContentArtist()

@param {Object} metadata
  A Cast metadata object, one of:
   - https://developers.google.com/cast/docs/reference/messages#GenericMediaMetadata
   - https://developers.google.com/cast/docs/reference/messages#MovieMediaMetadata
   - https://developers.google.com/cast/docs/reference/messages#TvShowMediaMetadata
   - https://developers.google.com/cast/docs/reference/messages#MusicTrackMediaMetadata
@export