[Icons NTP] Make Local NTP render largeIconUrl instead of thumbnail, if available.

chrome/browser/resources/local_ntp/most_visited_thumbnail.css

 /* Copyright 2013 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 {
   height: 100%;
   position: absolute;
   width: 100%;
 }
 
 a {
@@ skipping 17 matching lines @@
   width: 90%;
 }
 
 span.blocker {
   display: inline-block;
   height: 100%;
   position: absolute;
   width: 100%;
 }
 
-img {
+img.thumbnail {
   height: auto;
   min-height: 100%;
   width: 100%;
 }
+
+img.large-icon {
+  height: 48px;
+  left: 50%;
+  margin-left: -24px;
+  margin-top: -24px;
+  min-height: initial;
+  position: absolute;
+  top: 50%;
+  vertical-align: middle;
+  width: 48px;
+}