(WIP) Use generator to generate externs for chrome.fileManagerPrivate API.

ui/file_manager/file_manager/foreground/js/ui/gear_menu.js

Index: ui/file_manager/file_manager/foreground/js/ui/gear_menu.js
diff --git a/ui/file_manager/file_manager/foreground/js/ui/gear_menu.js b/ui/file_manager/file_manager/foreground/js/ui/gear_menu.js
index 8807c5c19321d1b8f14433a34897bf6916be7318..d24dbe2d4c6d457ab0a3c0614b8b6e95c784063e 100644
--- a/ui/file_manager/file_manager/foreground/js/ui/gear_menu.js
+++ b/ui/file_manager/file_manager/foreground/js/ui/gear_menu.js
@@ -64,7 +64,7 @@ function GearMenu(element) {
 
   /**
    * Volume space info.
-   * @type {Promise.<MountPointSizeStats>}
+   * @type {Promise.<chrome.fileManagerPrivate.MountPointSizeStats>}
    * @private
    */
   this.spaceInfoPromise_ = null;
@@ -75,8 +75,8 @@ function GearMenu(element) {
 }
 
 /**
- * @param {Promise.<MountPointSizeStats>} spaceInfoPromise Promise to be
- *     fulfilled with space info.
+ * @param {Promise.<chrome.fileManagerPrivate.MountPointSizeStats>}
+ *     spaceInfoPromise Promise to be fulfilled with space info.
  * @param {boolean} showLoadingCaption Whether show loading caption or not.
  */
 GearMenu.prototype.setSpaceInfo = function(