Make wallpaper picker manifest and thumbnails available when offline.

chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_images_grid.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.
 
 cr.define('wallpapers', function() {
   /** @const */ var ArrayDataModel = cr.ui.ArrayDataModel;
   /** @const */ var Grid = cr.ui.Grid;
   /** @const */ var GridItem = cr.ui.GridItem;
   /** @const */ var GridSelectionController = cr.ui.GridSelectionController;
   /** @const */ var ListSingleSelectionModel = cr.ui.ListSingleSelectionModel;
@@ skipping 14 matching lines @@
     return el;
   }
 
   WallpaperThumbnailsGridItem.prototype = {
     __proto__: GridItem.prototype,
 
     /** @override */
     decorate: function() {
       GridItem.prototype.decorate.call(this);
       var imageEl = cr.doc.createElement('img');
-      var xhr = new XMLHttpRequest();
-      xhr.open('GET', this.dataItem.baseURL + ThumbnailSuffix, true);
-      xhr.responseType = 'blob';
-      xhr.send(null);
       var self = this;
-      xhr.addEventListener('load', function(e) {
-        if (xhr.status === 200) {
-          self.textContent = '';
-          imageEl.src = window.URL.createObjectURL(xhr.response);
+      chrome.wallpaperPrivate.getThumbnail(this.dataItem.baseURL,
+                                           function(data) {
+        if (data) {
+          var blob = new Blob([new Int8Array(data)]);
+          imageEl.src = window.URL.createObjectURL(blob);
           imageEl.addEventListener('load', function(e) {
             window.URL.revokeObjectURL(this.src);
           });
           self.appendChild(imageEl);
+        } else {
+          var xhr = new XMLHttpRequest();
+          xhr.open('GET', self.dataItem.baseURL + ThumbnailSuffix, true);
+          xhr.responseType = 'arraybuffer';
+          xhr.send(null);
+          xhr.addEventListener('load', function(e) {
+            if (xhr.status === 200) {
+              self.textContent = '';
+              chrome.wallpaperPrivate.saveThumbnail(self.dataItem.baseURL,
+                                                    xhr.response);
+              var blob = new Blob([new Int8Array(xhr.response)]);
+              imageEl.src = window.URL.createObjectURL(blob);
+              // TODO(bshe): We currently use empty div to reserve space for
+              // thumbnail. Use a placeholder like "loading" image may better.
+              imageEl.addEventListener('load', function(e) {
+                window.URL.revokeObjectURL(this.src);
+              });
+              self.appendChild(imageEl);
+            }
+          });
         }
       });
     },
   };
 
   /**
    * Creates a selection controller that wraps selection on grid ends
    * and translates Enter presses into 'activate' events.
    * @param {cr.ui.ListSelectionModel} selectionModel The selection model to
    *     interact with.
@@ skipping 100 matching lines @@
       this.columns = 0;
       this.redraw();
       this.focus();
     }
   };
 
   return {
     WallpaperThumbnailsGrid: WallpaperThumbnailsGrid
   };
 });