Bug 1660359 part 3: Give the print preview browser an a11y label. r=mstriemer,fluent-reviewers,flod,jwatt

This is reported by a11y tools when the print preview has focus.

Differential Revision: https://phabricator.services.mozilla.com/D87998

toolkit/components/printing/content/print.js

@@ -155,6 +155,8 @@ var PrintEventHandler = {
       })
     );
 
+    await document.l10n.translateElements([this.previewBrowser]);
+
     document.body.removeAttribute("loading");
   },
 
@@ -174,6 +176,7 @@ var PrintEventHandler = {
     printPreviewBrowser.classList.add("printPreviewBrowser");
     printPreviewBrowser.setAttribute("flex", "1");
     printPreviewBrowser.setAttribute("printpreview", "true");
+    document.l10n.setAttributes(printPreviewBrowser, "printui-preview-label");
 
     // Create the stack for the loading indicator.
     let ourBrowser = window.docShell.chromeEventHandler;

toolkit/locales/en-US/toolkit/printing/printUI.ftl

@@ -72,6 +72,11 @@ printui-cancel-button = Cancel
 
 printui-loading = Preparing Preview
 
+# Reported by screen readers and other accessibility tools to indicate that
+# the print preview has focus.
+printui-preview-label =
+    .aria-label = Print Preview
+
 ## Paper sizes that may be supported by the Save to PDF destination:
 
 printui-paper-a5 = A5