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See bug(s) for spec. This CL adds an animated blur on wallpaper when entering/existing
overview when no maximized/fullscreen windows.

Bug: 823533, 812004
Change-Id: Icf8bdababedaaefe869f62768aacf7abcb3643d2
Reviewed-on: https://chromium-review.googlesource.com/979167Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549780}