Add missing type comments for file_manager_commands.js

ui/file_manager/file_manager/common/js/externs.js

Index: ui/file_manager/file_manager/common/js/externs.js
diff --git a/ui/file_manager/file_manager/common/js/externs.js b/ui/file_manager/file_manager/common/js/externs.js
index d5f375ee4e382ba80edcbf0a4c9dd6753f8aa25b..a96c5116c6516596b79a67249dccc29b70c439fc 100644
--- a/ui/file_manager/file_manager/common/js/externs.js
+++ b/ui/file_manager/file_manager/common/js/externs.js
@@ -18,6 +18,15 @@ BackgroundWindow.prototype.background;
  */
 BackgroundWindow.prototype.registerDialog = function(window) {};
 
+/**
+ * @param {Object=} opt_appState App state.
+ * @param {number=} opt_id Window id.
+ * @param {LaunchType=} opt_type Launch type. Default: ALWAYS_CREATE.
+ * @param {function(string)=} opt_callback Completion callback with the App ID.
+ */
+BackgroundWindow.prototype.launchFileManager =
+    function(opt_appState, opt_id, opt_type, opt_callback) {};
+
 
 /**
  * @constructor