ntp: remove default thumbnail for most visited (broken camera)

chrome/browser/resources/ntp4/most_visited_page.css

 /* Copyright (c) 2011 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.
  */
 
 .most-visited {
   position: absolute;
 }
 
 .most-visited {
@@ skipping 77 matching lines @@
   height: 23px;
   overflow: hidden;
   padding-top: 8px;
   text-align: center;
   text-overflow: ellipsis;
   white-space: nowrap;
 }
 
 .thumbnail {
   background: no-repeat;
+  /* This shows for missing thumbnails. */
+  background-color: #eee;
   background-size: 100%;
   border-radius: 3px;
   /* These max dimensions are not necessary, as the sizing logic in the .js
    * should be sufficient, but they're here for extra insurance. We never want
    * to scale a thumbnail larger than this size. */
   max-height: 132px;
   max-width: 212px;
   -webkit-transition: opacity 150ms;
 }
 
@@ skipping 54 matching lines @@
  * Override opacity of the tile to 1, so that the new tile animation
  * occurs simultaneously with the trash animation. */
 .tile.dragging.finishing-drag {
   opacity: 1;
 }
 
 /* Don't display the new tile until there's something to show.  */
 .blacklisted {
   opacity: 0;
 }