A11y: Set role="alert" in all cr-toast.

Bug: 1015194
Change-Id: I71947da1d8f77365244496754518d1d722c234d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1912802
Auto-Submit: Hector Carmona <hcarmona@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715029}

ui/webui/resources/cr_elements/cr_toast/cr_toast.js

@@ -20,6 +20,10 @@ Polymer({
     },
   },
 
+  hostAttributes: {
+    'role': 'alert',
+  },
+
   observers: ['resetAutoHide_(duration, open)'],
 
   /** @private {number|null} */

ui/webui/resources/cr_elements/cr_toast/cr_toast_manager.html

@@ -30,7 +30,7 @@
         white-space: nowrap;
       }
     </style>
-    <cr-toast id="toast" duration="[[duration]]" role="alert">
+    <cr-toast id="toast" duration="[[duration]]">
       <div id="content" class="elided-text"></div>
       <cr-button id="button" hidden$="[[!showUndo_]]" on-click="onUndoClick_"
           aria-label$="[[undoDescription]]">