[CrOS Wallpaper] Fix wrong tab focus on wallpaper app after previewing.

Currently, when you enter and exit the preview mode, the list items
(each wallpaper thumbnail in the current collection) lose their tabIndex
and aria-label.  This CL adds these properties back in so that the user
can tab through and the title is read out in chromevox.

Bug: 1005633
Change-Id: I60a2b2b5ea714d37bac440e9327d95b4867bc112
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1888377Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710919}

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

@@ -713,12 +713,16 @@ cr.define('wallpapers', function() {
         var scrollUp =
             this.cachedScrollTop_ && this.cachedScrollTop_ > this.scrollTop;
         for (var i = 0; i < this.dataModel.length; ++i) {
-          if (this.getListItemByIndex(i)) {
-            this.getListItemByIndex(i).classList.toggle(
+          const listItem = this.getListItemByIndex(i);
+          if (listItem) {
+            listItem.classList.toggle(
                 'first-row',
                 i < this.columns &&
                     (this.firstIndex_ == 0 || i != this.firstIndex_ ||
                      scrollUp));
+            listItem.tabIndex = 0;
+            listItem.setAttribute(
+                'aria-label', this.dataModel.item(i).ariaLabel);
           }
         }
       }