Make printer configuration toasts alert via ChromeVox

-Currently when a user has ChromeVox turned on and a printer is
 successfully added, a success toast pops up but ChromeVox does not
 announce it. This change forces ChromeVox to read out the contents of
 the toast.

Bug: 987343
Change-Id: I9ffa73d8765ddfbdaad421bf13d3ab4efaaadfef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1850250Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Commit-Queue: Gavin Williams <gavinwill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705179}

chrome/browser/resources/settings/printing_page/cups_printers.html

@@ -197,7 +197,7 @@
       </settings-cups-edit-printer-dialog>
     </template>
 
-    <cr-toast id="errorToast" duration="3000">
+    <cr-toast id="errorToast" duration="3000" role="alert">
       <div class="error-message" id="addPrinterDoneMessage">
         [[addPrinterResultText_]]
       </div>