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The wallpaper paint code of the centered layout used to just
draw the image with its original size centered in the display
area. DeskMiniView which uses WallpaperBaseView to draw the
wallpaper in its small bounds needs to apply a scale to that
original size of the wallpaper image, to scale it down by the
same factor it scales down the desk.

Demo: https://bugs.chromium.org/p/chromium/issues/detail?id=992339#c6

BUG=992339
TEST=Manually, Pick a wallpaper image from the local file system,
     and make its layout centered.

Change-Id: I28e253c8ba48b12a25c83576e41037d1ad24783f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1896245Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712162}