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Partial revert of https://chromium-review.googlesource.com/c/chromium/src/+/1481899.
It turns out that that change did not remove a full screen blit, so it
was a lateral change at best. But that cl does fix another wallpaper bug
and left some framework for the next change, which confirm to remove a
fullscreen blit, but will not make it in time for m74.

TBR=xiyuan@chromium.org

Test: none
Bug: 774690
Change-Id: I3a42d9a2dfdc81b848998858ea4fad0218c7f7a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506372
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638285}