[MD] Change appearance of incognito NTP to MD spec.

chrome/browser/resources/ntp4/new_incognito_tab_theme.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.
  */
 
 html {
   background-attachment: fixed;
   background-color: ${colorBackground};
-  background-image: url(chrome://theme/IDR_THEME_NTP_BACKGROUND?${themeId});
   background-position: ${backgroundBarDetached};
   background-repeat: ${backgroundTiling};
   height: 100%;
   overflow: auto;
 }
 
+html${hasCustomBackground} {
+  background-image: url(chrome://theme/IDR_THEME_NTP_BACKGROUND?${themeId});
+}
+
+html${hasCustomBackground} .content {
+  border-radius: 2px;
+  box-shadow: 0 4px 6px 1px rgba(0, 0, 0, 0.4);
+}
+
 #attribution-img {
   content: url(chrome://theme/IDR_THEME_NTP_ATTRIBUTION?${themeId});
 }
 
 html[bookmarkbarattached='true'] {
   background-position: ${backgroundBarAttached};
 }