Extract colors from wallpaper and dynamically update login screen overlay

ui/login/md_screen_container.css

 /* Copyright 2014 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.
  */
 
 #outer-container {
   -webkit-box-align: center;
   -webkit-box-pack: center;
   bottom: 57px;  /* Leave space for the header bar */
   display: -webkit-box;
@@ skipping 46 matching lines @@
 #inner-container.animation {
   overflow: hidden;
 }
 
 #inner-container.disabled {
   opacity: 0.4;
   pointer-events: none;
 }
 
 #login-shield {
-  background-color: rgba(20, 29, 40, .8); /* #141D28 80% */
+  background-color: rgba(0, 0, 0, 0.6);
   display: block;
   height: 100%;
   left: 0;
   position: absolute;
   top: 0;
   width: 100%;
 }
 
 #oobe[md-mode] #progress-dots {
   display: none;
@@ skipping 157 matching lines @@
   line-height: 31px;
   text-transform: lowercase;
   width: 23em;
 }
 
 .header-section::before {
   /* Divider in header between product name and title,
    * like "[Product name] > [step header]". */
   content: '\00A0\203A\00A0\00A0';
 }