Make wallpaper picker manifest and thumbnails available when offline.

chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_images_grid.js

Index: chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_images_grid.js
diff --git a/chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_images_grid.js b/chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_images_grid.js
index 74e65edeb9fc9c2b4a371ab4b433d8541d606e78..130bc42c59523e63d97496441d4fd0eca9cf87be 100644
--- a/chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_images_grid.js
+++ b/chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_images_grid.js
@@ -32,19 +32,36 @@ cr.define('wallpapers', function() {
     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);
+            }
+          });
         }
       });
     },