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.

TBR=alemate@chromium.org

(cherry picked from commit 7967fd7d6a8fd9c7f7cee16ae6f9eeb5284dd0fb)

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-Original-Commit-Position: refs/heads/master@{#579977}
Reviewed-on: https://chromium-review.googlesource.com/1171946
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/branch-heads/3497@{#552}
Cr-Branched-From: 271eaf50594eb818c9295dc78d364aea18c82ea8-refs/heads/master@{#576753}
8 files changed