ntp4: Fix a couple small visual glitches with tile dragging.

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.
  *
  * This is the stylesheet used by the touch-enabled new tab page
  */
 
 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 */
   height: 100%;
   overflow: hidden;
 }
 
 body {
   background-size: auto 100%;
   margin: 0;
   /* Don't highlight links when they're tapped.  Safari has bugs here that
      show up as flicker when dragging in some situations */
   -webkit-tap-highlight-color: transparent;
   /* Don't allow selecting text - can occur when dragging */
   -webkit-user-select: none;
 }
 
+.hidden {
+  display: none !important;
+}
+
 #card-slider-frame {
   /* Must match #footer height. */
   bottom: 50px;
   overflow: hidden;
   /* We want this to fill the window except for the region used
      by footer */
   position: fixed;
   top: 0;
   width: 100%;
   -webkit-mask-image: -webkit-linear-gradient(bottom, transparent, black 30px);
@@ skipping 92 matching lines @@
 #footer.rearrange-mode #trash {
   -webkit-transform: translate(0, 0);
 }
 
 .disclosure-triangle {
   display: inline-block;
   width: 9px;
   height: 9px;
   -webkit-mask-image: url('./ntp4_disclosure_triangle_mask.png');
 }