MD Downloads: fix up some Polymer 1.2.3 issues and re-vulcanize

chrome/browser/resources/md_downloads/manager.css

 /* Copyright 2015 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. */
 
 :host {
   display: flex;
   flex: 1 0;
   flex-direction: column;
   height: 100%;
+  overflow-y: overlay;
 }
 
 @media screen and (max-width: 1024px) {
   :host {
     flex-basis: 670px;  /* 622 card width + 24 left margin + 24 right margin. */
   }
 }
 
-#panel {
-  --paper-header-panel-shadow: {
-    display: none;
-  };
-  --paper-header-panel-standard-container: {
-    display: flex;
-    overflow-y: overlay;
-  };
-}
-
 #no-downloads,
 #downloads-list {
   flex: 1;
 }
 
-.loading #no-downloads,
-.loading #downloads-list {
+:host([loading]) #no-downloads,
+:host([loading]) #downloads-list {
   display: none;
 }
 
 #no-downloads {
   align-items: center;
   color: #b4b4b4;
   display: flex;
   font-size: 123.1%;
   font-weight: 500;
   justify-content: center;
 }
 
 #no-downloads .illustration {
   background: -webkit-image-set(
       url(chrome://downloads/1x/no_downloads.png) 1x,
       url(chrome://downloads/2x/no_downloads.png) 2x) no-repeat center center;
   height: 144px;  /* Matches natural image height. */
   margin-bottom: 32px;
 }