Gallery: Add debugMe method to Gallery.

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-family: Noto Sans UI, sans-serif;
   font-size: 84%;
   margin: 0;
 }
@@ skipping 1322 matching lines @@
       url(chrome://resources/images/2x/apps/topbar_button_maximize.png) 2x)
       center;
 }
 
 .gallery > .header > .close-button {
   background: -webkit-image-set(
       url(chrome://resources/images/apps/topbar_button_close.png) 1x,
       url(chrome://resources/images/2x/apps/topbar_button_close.png) 2x)
       center;
 }
+
+.debug-me .load-target-content-metadata::before,
+.debug-me .load-target-external-metadata::before,
+.debug-me .load-target-file-entry::before {
+  bottom: 0;
+  content: '';
+  display: block;
+  left: 0;
+  position: absolute;
+  right: 0;
+  top: 0;
+  z-index: 1;
+}
+
+.debug-me .load-target-content-metadata::before {
+  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);
+}