cros: Fix empty preview wallpaper description issue

Bug: 873340
Change-Id: Ia5979ba5305b154a27689157db573f8bc6803801
Reviewed-on: https://chromium-review.googlesource.com/1171790Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582407}

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

@@ -1548,10 +1548,9 @@ WallpaperManager.prototype.onFileSystemError_ = function(e) {
  * Handles changing of selectedItem in wallpaper manager.
  */
 WallpaperManager.prototype.onSelectedItemChanged_ = function() {
-  this.setWallpaperAttribution(this.selectedItem_);
-
   if (!this.selectedItem_ || this.selectedItem_.source == 'ADDNEW')
     return;
+  this.setWallpaperAttribution(this.selectedItem_);
 
   if (this.selectedItem_.baseURL &&
       (this.useNewWallpaperPicker_ ||