[Local NTP] block image-related context menu items for thumbnails; CSS fixes.

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 {
   height: 100%;
+  position: relative;
   width: 100%;
 }
 
 a:focus {
   outline: none;  /* Remove outline from tabIndex = -1. */
 }
 
 div {
   bottom: 24px;
   margin: 0 7px;
   overflow: hidden;
   position: absolute;
   text-align: center;
   text-overflow: ellipsis;
   white-space: nowrap;
   width: 90%;
 }
 
+span.blocker {
+  display: inline-block;
+  height: 100%;
+  position: absolute;
+  width: 100%;
+}
+
 img {
   height: 100%;
   width: 100%;
 }