Add support for favicon scale factor in WebUI

chrome/browser/resources/ntp4/most_visited_page.css

 /* Copyright (c) 2012 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. */
 
 .most-visited {
   position: absolute;
   z-index: 0;
 }
 
 .most-visited {
@@ skipping 45 matching lines @@
   opacity: 1;
 }
 
 .most-visited .close-button:hover {
   -webkit-transition: none;
 }
 
 .most-visited .favicon {
   -webkit-margin-start: 5px;
   background: no-repeat left 50%;
+  background-size: 16px;
   bottom: 7px;
   box-sizing: border-box;
   display: block;
   height: 16px;
   position: absolute;
   width: 16px;
 }
 
 html[dir='rtl'] .most-visited .favicon {
   background-position-x: right;
@@ skipping 100 matching lines @@
  * Override opacity of the tile to 1, so that the new tile animation
  * occurs simultaneously with the trash animation. */
 .tile.dragging.finishing-drag {
   opacity: 1;
 }
 
 /* Don't display the new tile until there's something to show.  */
 .blacklisted {
   opacity: 0;
 }