Fix trivial type-check errors in file_manager.

ui/file_manager/file_manager/background/js/background.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 /**
  * Type of a Files.app's instance launch.
  * @enum {number}
  */
@@ skipping 84 matching lines @@
    * It turns to null after initialization.
    *
    * @type {Array.<function()>}
    * @private
    */
   this.initializeCallbacks_ = [];
 
   /**
    * Last time when the background page can close.
    *
-   * @type {number}
+   * @type {?number}
    * @private
    */
   this.lastTimeCanClose_ = null;
 
   // Seal self.
   Object.seal(this);
 
   // Initialize handlers.
   chrome.fileBrowserHandler.onExecute.addListener(this.onExecute_.bind(this));
   chrome.app.runtime.onLaunched.addListener(this.onLaunched_.bind(this));
@@ skipping 76 matching lines @@
     setTimeout(this.tryClose.bind(this), FileBrowserBackground.CLOSE_DELAY_MS_);
     return false;
   }
 
   // Otherwise we can close the background page.
   return true;
 };
 
 /**
  * Opens the root directory of the volume in Files.app.
- * @param {string} volumeId ID of a volume to be opened.
+ * @param {string} devicePath Device path to a volume to be opened.
  * @private
  */
 FileBrowserBackground.prototype.navigateToVolume = function(devicePath) {
   VolumeManager.getInstance().then(function(volumeManager) {
     var volumeInfoList = volumeManager.volumeInfoList;
     for (var i = 0; i < volumeInfoList.length; i++) {
       if (volumeInfoList.item(i).devicePath == devicePath)
         return volumeInfoList.item(i).resolveDisplayRoot();
     }
     return Promise.reject(
         'Volume having the device path: ' + devicePath + ' is not found.');
   }).then(function(entry) {
     launchFileManager(
         {currentDirectoryURL: entry.toURL()},
-        /* App ID */ null,
+        /* App ID */ undefined,
         LaunchType.FOCUS_SAME_OR_CREATE);
   }).catch(function(error) {
     console.error(error.stack || error);
   });
 };
 
 /**
  * Prefix for the file manager window ID.
  * @type {string}
  * @const
@@ skipping 187 matching lines @@
     // the selectionURL only, and the currentDirectory will be resolved
     // later.
     selectionURL: details.entries[0].toURL()
   };
 
   // Every other action opens a Files app window.
   // For mounted devices just focus any Files.app window. The mounted
   // volume will appear on the navigation list.
   launchFileManager(
       appState,
-      /* App ID */ null,
+      /* App ID */ undefined,
       LaunchType.FOCUS_SAME_OR_CREATE);
 };
 
 /**
  * Launches the app.
  * @private
  */
 FileBrowserBackground.prototype.onLaunched_ = function() {
   if (nextFileManagerWindowID == 0) {
     // The app just launched. Remove window state records that are not needed
     // any more.
     chrome.storage.local.get(function(items) {
       for (var key in items) {
         if (items.hasOwnProperty(key)) {
           if (key.match(FILES_ID_PATTERN))
             chrome.storage.local.remove(key);
         }
       }
     });
   }
-  launchFileManager(null, null, LaunchType.FOCUS_ANY_OR_CREATE);
+  launchFileManager(null, undefined, LaunchType.FOCUS_ANY_OR_CREATE);
 };
 
 /**
  * Restarted the app, restore windows.
  * @private
  */
 FileBrowserBackground.prototype.onRestarted_ = function() {
   // Reopen file manager windows.
   chrome.storage.local.get(function(items) {
     for (var key in items) {
       if (items.hasOwnProperty(key)) {
         var match = key.match(FILES_ID_PATTERN);
         if (match) {
           var id = Number(match[1]);
           try {
-            var appState = JSON.parse(items[key]);
+            var appState = /** @type {Object} */ (JSON.parse(items[key]));
             launchFileManager(appState, id);
           } catch (e) {
             console.error('Corrupt launch data for ' + id);
           }
         }
       }
     }
   });
 };
 
@@ skipping 48 matching lines @@
     contexts: ['launcher'],
     title: str('NEW_WINDOW_BUTTON_LABEL')
   });
 };
 
 /**
  * Singleton instance of Background.
  * @type {FileBrowserBackground}
  */
 window.background = new FileBrowserBackground();