overview: Prevent wallpaper from flashing when exit and reenter quickly.

Readded a piece of code i accidently removed in a previous patch.

Test: manual
Bug: 944688
Change-Id: Ic998e38c1a93cfd2428d8244517865d7984ea514
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1534643
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643376}
1 file changed