Move bbox property to GeoJSONObject

See http://geojson.org/geojson-spec.html#geojson-objects:
"A GeoJSON object may have a "bbox" member [...]"

externs/geojson.js

@@ -13,6 +13,12 @@
 var GeoJSONObject = function() {};
 
 
+/**
+ * @type {!Array.<number>|undefined}
+ */
+GeoJSONObject.prototype.bbox;
+
+
 /**
  * @type {string}
  */
@@ -110,12 +116,6 @@ GeoJSONGeometryCollection.prototype.geometries;
 var GeoJSONFeature = function() {};
 
 
-/**
- * @type {!Array.<number>|undefined}
- */
-GeoJSONFeature.prototype.bbox;
-
-
 /**
  * @type {GeoJSONGeometry}
  */
@@ -148,12 +148,6 @@ var GeoJSONFeatureCollection = function() {};
 GeoJSONFeatureCollection.prototype.features;
 
 
-/**
- * @type {!Array.<number>|undefined}
- */
-GeoJSONFeatureCollection.prototype.bbox;
-
-
 
 /**
  * @constructor