Add a frameState property to ol.MapBrowserEvent

so that we will be able to retrieve the current center of the view2d from the
framestate in the dragpan interaction.

src/ol/map.js

@@ -542,6 +542,7 @@ ol.Map.prototype.handleControlsRemove_ = function(collectionEvent) {
  * @param {ol.MapBrowserEvent} mapBrowserEvent The event to handle.
  */
 ol.Map.prototype.handleMapBrowserEvent = function(mapBrowserEvent) {
+  mapBrowserEvent.frameState = this.frameState_;
   var interactions = this.getInteractions();
   var interactionsArray = /** @type {Array.<ol.interaction.Interaction>} */
       (interactions.getArray());