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This also fixes the blur on this new wallpaper.

Plus, change AddRemoveDisplay to use the same display id
for added display.

Fixed: 1126186
Test: covered by unittest

Change-Id: I2698d8f073f58edb18e29e84e7e16c4296ea48a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2425431
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810275}