Print Preview: Fix WebUI Event name

Incorrect event name was used when registering the listener, so the
event was never received.

Bug: 753942
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ie58dacc705632f90857bf8fb19139d7f967e1684
Reviewed-on: https://chromium-review.googlesource.com/609093
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#493243}