overview: Set windows event targeting policy to NONE.

This will allow events to just pass through the window and drop down to
item_widget_. item_widget_ is stacked on top of window normally, to
prevent events from reaching the window.

With this change, we can keep window stacked on top of item_widget_.
This allow us to make backdrop_widget_ part of item_widget_ in the
future and then possible item_widget_ part of shield_widget_ to get
rid of some uneeded widgets.

Test: manual
Bug: 914147
Change-Id: I2e227a17d56cf7fcd121bdf108e74b0d5b3bcc15
Reviewed-on: https://chromium-review.googlesource.com/c/1464321
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631789}
5 files changed