refactor arrow props code

Author: chrisbreiding

src/portal-popper.jsx

@@ -103,15 +103,16 @@ class PortalPopper extends Component {
   }
 
   _getArrowStyle () {
-    const left = _.isNumber(this.state.arrowProps.left) ? Math.round(this.state.arrowProps.left) : null
-    const top = _.isNumber(this.state.arrowProps.top) ? Math.round(this.state.arrowProps.top) : null
-
     return {
-      left,
-      top,
+      left: this._getArrowProp('left'),
+      top: this._getArrowProp('top'),
     }
   }
 
+  _getArrowProp (position) {
+    return _.isNumber(this.state.arrowProps[position]) ? Math.round(this.state.arrowProps[position]) : null
+  }
+
   componentWillUnmount () {
     this.isUnmounted = true
     this.popper && this.popper.destroy()