overview: Fix bug which item did not receive events when have backdrop.

The backdrop is created after the shield button and it's the same size
so it was eating all the events. Set the backdrop to not be able to
handle events.

Test: ash_unittests OverviewSessionTest.SelectingWindowWithBackdrop
Bug: 938645
Change-Id: I18d53cf9ba385ab24a7e3dcce1ceaf16fcde48d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504307
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638238}
2 files changed