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It was disabled because causing a regression. Switches it up a bit to
instead create a smaller widget which draws its own blurred
WallpaperView, which is then scaled before being faded in. The widget
is kept alive for the duration of overview.

Test: manual
Bug: 1012829
Change-Id: I0b132a52c2ea26ffddd6b6537ab4062b80960e7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876873
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711321}