Add missing type comments for file_manager_commands.js

BUG=406995
TEST=None

Review URL: https://codereview.chromium.org/691273004

Cr-Commit-Position: refs/heads/master@{#302241}

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

ui/file_manager/file_manager/foreground/js/ui/file_manager_ui.js

@@ -7,7 +7,8 @@
  *
  * @param {HTMLElement} element Top level element of Files.app.
  * @param {DialogType} dialogType Dialog type.
- * @constructor.
+ * @constructor
+ * @struct
  */
 function FileManagerUI(element, dialogType) {
   /**