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Keeps this behind a flag, support for old wallpaper blur still exists.
Copies the wallpaper layer to a layer which is blurred and kept alive
for the duration of overview.

Test: manual
Bug: 1012829
Change-Id: I74046bb522d0751304cadd5aa5dd361f70b2831c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1842698Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704977}