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

chrome/browser/resources/md_downloads/toolbar.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 {
   align-items: center;
   background: rgb(63, 85, 102);
   color: white;
   content-sizing: padding-box;
   display: flex;
   min-height: 56px;
 }
 
 #title h1 {
   -webkit-margin-end: 0;
   -webkit-margin-start: 24px;
   font-size: 107.7%;
   font-weight: normal;
   margin-bottom: 0;
   margin-top: 0;
 }
 
 #actions {
   display: flex;
   flex: none;
   width: var(--downloads-item-width);
 }
 
-:host-context(.loading) #actions {
+:host-context([loading]) #actions {
   visibility: hidden;
 }
 
 :host(:not([downloads-showing])) #actions {
   justify-content: center;
 }
 
 #actions paper-button:first-of-type {
   -webkit-margin-start: -0.57em;  /* Matches paper-button padding. */
 }
@@ skipping 63 matching lines @@
     display: none;
   }
 }
 
 @media all and (max-width: 1024px) {
   /* Hide #actions for narrow windows; they're shown in a vertical dot menu. */
   #actions {
     display: none;
   }
 }