Gallery: Add items to GalleryDataModel before loading their metadata.

ui/file_manager/gallery/css/gallery.css

 /* Copyright (c) 2014 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. */
 
 body {
   -webkit-user-select: none;
   font-size: 84%;
   margin: 0;
 }
 
@@ skipping 423 matching lines @@
 
 .gallery .ribbon-spacer {
   height: 55px;
   margin-bottom: -55px;
 }
 
 .gallery .toolbar .ribbon {
   height: 100%;
   overflow: hidden;
   transition: opacity 180ms linear, visibility 0ms linear;
+  white-space: nowrap;
   z-index: 0;
 }
 
 .gallery[editing] .toolbar .ribbon {
   opacity: 0;
   transition-delay: 0ms, 180ms;
   visibility: hidden;
 }
 
 .gallery .ribbon-image {
@@ skipping 902 matching lines @@
   background-color: rgba(255, 0, 0, 0.3);
 }
 
 .debug-me .load-target-external-metadata::before {
   background-color: rgba(0, 255, 0, 0.3);
 }
 
 .debug-me .load-target-file-entry::before {
   background-color: rgba(0, 0, 255, 0.3);
 }