Unified: Add Guest tooltip to the user avatar.

The user avatar did not have proper tooltip when it's guest session.

TEST=manual
BUG=889831

Change-Id: I620704000b2d01b83406eeebc301c54a4138bac1
Reviewed-on: https://chromium-review.googlesource.com/1250383Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595470}

ash/system/unified/user_chooser_view.cc

@@ -147,6 +147,9 @@ base::string16 GetUserItemAccessibleString(int user_index) {
       Shell::Get()->session_controller()->GetUserSession(user_index);
   DCHECK(user_session);
 
+  if (user_session->user_info->type == user_manager::USER_TYPE_GUEST)
+    return l10n_util::GetStringUTF16(IDS_ASH_STATUS_TRAY_GUEST_LABEL);
+
   return l10n_util::GetStringFUTF16(
       IDS_ASH_STATUS_TRAY_USER_INFO_ACCESSIBILITY,
       base::UTF8ToUTF16(user_session->user_info->display_name),