Remove window.event support for V1 ShadowDOM

window.event should be undefined if an Event's target is in a shadow
tree, because it is leaking information from inside the shadow tree.

This change will remove window.event support for events targeted to
nodes within a V1 shadow tree.

Discussion link: https://github.com/whatwg/dom/issues/334

Bug: 779461
Change-Id: I310bc72463369befd693de9321a9949a97e83f5e
Reviewed-on: https://chromium-review.googlesource.com/773922


Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518317}