ChromeOS OOBE: Make sure all oobe-dialog receive onBeforeShow event.

This CL routes onBeforeShow event to the oobe-dialogs, that are not subnodes
of another Polymer object.

Bug: 828999
Change-Id: Id2cee750d2364e53052133bbea7d27ea2c1ca0a2
Reviewed-on: https://chromium-review.googlesource.com/1157912
Reviewed-by: Yury Khmel <khmel@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579977}
8 files changed