Mark un and pw fields as optional in add SMB share dialog

- Marks username and password as optional in the add share dialog.

Bug: chromium:891020
Change-Id: Ie3ea8c2a021dd6be42a129865aac30fe00f67dcb
Reviewed-on: https://chromium-review.googlesource.com/1255900Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595645}

chrome/app/settings_strings.grdp

@@ -1424,10 +1424,10 @@
       Display name (optional)
     </message>
     <message name="IDS_SETTINGS_DOWNLOADS_ADD_SHARE_USERNAME" desc="Title for the input that lets users specify their username for an SMB share.">
-      Username
+      Username (optional)
     </message>
     <message name="IDS_SETTINGS_DOWNLOADS_ADD_SHARE_PASSWORD" desc="Title for the input that lets users specify their password for an SMB Share.">
-      Password
+      Password (optional)
     </message>
     <message name="IDS_SETTINGS_DOWNLOADS_SHARE_ADDED_SUCCESS_MESSAGE" desc="The message shown when a new SMB share is successfully mounted.">
       Share mounted successfully.