option to only sync when connected to wifi / metered network

[jimmywarting]

Going to reference this issue: WICG/background-fetch#172

Essentially: I only want to download songs for offline use when i'm connected to internet with unlimited data plan (otherwise i will play already downloaded songs that i have stored already)

triggering sync when being online or periodic dose not cut it for me... i want to be friendly towards the clients data cost and save money

/*
ConnectionType =
  | 'bluetooth'
  | 'cellular'
  | 'ethernet'
  | 'mixed'
  | 'none'
  | 'other'
  | 'unknown'
  | 'wifi'
  | 'wimax';
*/

const request = new Request('/upload', { body, method: 'POST' })

registration.sync.register('download-song', {
  // possible types: http://wicg.github.io/netinfo/#connection-types
  onlyOnConnectionType: ['wifi', 'ethernet'],
  onlyOnMetered: true
})