Added onCancel implementation

Author: gautamdsheth

src/controls/filePicker/FilePicker.tsx

@@ -206,6 +206,9 @@ export class FilePicker extends React.Component<IFilePickerProps, IFilePickerSta
    * Closes the panel
    */
   private _handleClosePanel = () => {
+    if (this.props.onCancel !== undefined) {
+      this.props.onCancel();
+    }
     this.setState({
       panelOpen: false
     });