[Nearby] ARIA live updates on receive flow.

This gives the content container of the receive dialog an aria-live
attribute so that when we change from the receive page to the confirm
page ChromeVox will begin reading the contents of the confirm page.

Fixed: 1149743
Change-Id: I58a6aeec0d99e0f4a9da67d68483f014b2ee4864
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2561649Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Cr-Commit-Position: refs/heads/master@{#831227}

chrome/browser/resources/settings/chromeos/nearby_share_page/nearby_share_receive_dialog.html

@@ -21,7 +21,7 @@
       }
     </style>
     <cr-dialog id="dialog" show-on-attach>
-      <div id="content" slot="body">
+      <div id="content" slot="body" aria-live="polite">
         <cr-view-manager id="viewManager">
           <nearby-share-high-visibility-page id="[[Page.HIGH_VISIBILITY]]"
               slot="view" device-name="[[settings.deviceName]]">