title: Firebase messaging - whats "content_available" : true
body: <p>I saw many examples of firebase push like </p>
  
  <pre><code>{ 
    "to" : "egu9jGiMcew:APA91bFv2Rewdz.....KZZmEURvbq-aA2", 
    "data": { 
      "id": 19, 
      "title": "Title test", 
      "msg": "Text of the test", 
      "code": 2, 
      "infosUrl": "ttp://www.secondguide.tours", 
      "longitude": 5.5, 
      "latitude": 44.5, 
      "eventLocation": "nowhere", 
      "startDate": "24/07/2016", 
      "endDate": "25/07/2016", 
    }, 
    "delay_while_idle" : false, 
    "priority" : "high", 
    "content_available" : true 
  } 
  </code></pre>
  
  <p>I did not get the purpose of      </p>
  
  <pre><code>"delay_while_idle" : false,    
  "priority" : "high",   
  "content_available" : true
  </code></pre>
  
  <p>in this, can somebody explain?</p>