title: How do I get the bounding box of a mapboxgl.GeoJSONSource object?
body: <p>I'm setting up a Mapbox GL JS map like this:</p>
  
  <pre><code>mapboxgl.accessToken = 'pk.my_token';
  var cityBoundaries   = new mapboxgl.GeoJSONSource({ data: 'http://domain.com/city_name.geojson' } );
  var map              = new mapboxgl.Map({
    container: 'map',
    style: 'mapbox://styles/mapbox/streets-v8',
    center: [cityLongitude,cityLatitude],
    zoom: 13
  });
  </code></pre>
  
  <p>Then I'm loading that GeoJSON data onto the map after it loads like this:</p>
  
  <pre><code>map.on('style.load', function(){
    map.addSource('city', cityBoundaries);
    map.addLayer({
      'id': 'city',
      'type': 'line',
      'source': 'city',
      'paint': {
        'line-color': 'blue',
        'line-width': 3
      }
    });
  });
  </code></pre>
  
  <p>At this point, I have a map that's centered at the location I specified in <code>new mapboxgl.Map</code>, and it's at zoom level 13. So, only a piece of the GeoJSON data is visible on the map. I'd like to re-center and re-zoom the map so that the entire GeoJSON data is visible.</p>
  
  <p>In Mapbox JS, I would do this by loading the GeoJSON data into a <code>featureLayer</code> and then fitting the map to its bounds with:</p>
  
  <pre><code>map.fitBounds(featureLayer.getBounds());
  </code></pre>
  
  <p>The <a href="https://www.mapbox.com/mapbox-gl-js/api/#Map.fitBounds" rel="noreferrer">fitBounds documentation</a> for Mapbox GL JS indicates that it wants the bounds in the format of <code>[[minLng, minLat], [maxLng, maxLat]]</code>.</p>
  
  <p>Is there a way to determine the mix/max latitude &amp; longitude values of this GeoJSON layer?</p>