Disable bookmark pane in ntp4.

chrome/browser/resources/ntp4/new_tab.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.
  */
 
 html {
   font-family: segoe ui, arial, helvetica, sans-serif;
   font-size: 14px;
   /* It's necessary to put this here instead of in body in order to get the
      background-size of 100% to work properly */
@@ skipping 206 matching lines @@
   min-width: 90px;
   top: 0;
   z-index: 5;
   -webkit-transition: width 150ms, right 150ms, background-color 150ms;
 }
 
 .page-switcher:hover,
 .page-switcher.drag-target {
   background-color: rgba(0, 0, 0, 0.2);
 }
+
+html[dir=ltr] #page-switcher-start,
+html[dir=rtl] #page-switcher-end {
+  border-top-right-radius: 10px;
+}
+
+html[dir=rtl] #page-switcher-start,
+html[dir=ltr] #page-switcher-end {
+  border-top-left-radius: 10px;
+}