Chrome bookmarks toast - adding undo shortcut description as aria-label.

This change adds an aria-label to the "undo" button of the form
"press Control plus Z to undo".
It will appropriately insert either "Control" or "Command" depending on
platform.

BUG=856209

Change-Id: I95f917b8d6722ad8362dea2295bfc4aaf82f6259
Reviewed-on: https://chromium-review.googlesource.com/c/1229737
Commit-Queue: Chris Hall <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: calamity <[email protected]>
Cr-Commit-Position: refs/heads/master@{#598682}
diff --git a/components/undo_strings.grdp b/components/undo_strings.grdp
index 8183e5d..f1380ca 100644
--- a/components/undo_strings.grdp
+++ b/components/undo_strings.grdp
@@ -1,6 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <grit-part>
 
+  <message name="IDS_BOOKMARK_BAR_UNDO_DESCRIPTION" desc="Shortcut description for undoing a bookmark operation, used as a hint for keyboard users">
+    press <ph name="MODIFIER_KEY_DESCRIPTION">$1<ex>Control Z</ex></ph> to undo
+  </message>
+
   <if expr="not use_titlecase">
     <message name="IDS_BOOKMARK_BAR_UNDO" desc="Menu title for undoing a bookmark operation">
       &amp;Undo