MD Bookmarks: Initial screenreader accessibility improvements

chrome/browser/resources/md_bookmarks/toast_manager.html

 <link rel="import" href="chrome://resources/html/polymer.html">
-
+<link rel="import" href="chrome://resources/polymer/v1_0/iron-a11y-announcer/iron-a11y-announcer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://bookmarks/shared_style.html">
 
 <dom-module id="bookmarks-toast-manager">
   <template>
     <style include="shared-style">
       #content {
         display: flex;
         flex: 1;
         margin-right: 32px;
@@ skipping 34 matching lines @@
         min-height: 52px;
         min-width: 288px;
         opacity: 0;
         padding: 0 24px;
         position: fixed;
         transform: translateY(100px);
         transition: transform 300ms, opacity 300ms;
         white-space: nowrap;
       }
 
-      :host([open_]) #toast {
+      #toast[aria-hidden=false] {
         opacity: 1;
         transform: translateY(0);
       }
     </style>
-    <div id="toast" aria-live="polite">
+    <div id="toast">
       <div id="content"></div>
       <paper-button id="button" hidden$="[[!showUndo_]]" on-tap="onUndoTap_">
         $i18n{undo}
       </paper-button>
     </div>
   </template>
   <script src="chrome://bookmarks/toast_manager.js"></script>
 </dom-module>