Oobe: add hidden attribute in screen element for testing

Bug: 1145679
Change-Id: If4128d19c6bc6fe9b4c3960a22a521997d9c2d9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2558792Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Yunke Zhou <yunkez@google.com>
Cr-Commit-Position: refs/heads/master@{#830958}

chrome/browser/resources/chromeos/login/structure/screens_common.html

@@ -44,9 +44,9 @@
 </gesture-navigation-element>
 <marketing-opt-in-element id="marketing-opt-in" class="step hidden">
 </marketing-opt-in-element>
-<family-link-notice-element id="family-link-notice" class="step hidden">
+<family-link-notice-element id="family-link-notice" class="step hidden" hidden>
 </family-link-notice-element>
-<user-creation-element id="user-creation" class="step hidden">
+<user-creation-element id="user-creation" class="step hidden" hidden>
 </user-creation-element>
-<parental-handoff-element id="parental-handoff" class="step hidden">
+<parental-handoff-element id="parental-handoff" class="step hidden" hidden>
 </parental-handoff-element>