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The old wallpaper picker must generate and save a thumbnail for each
custom wallpaper, so that they can persist even after the original
image file is deleted. This is not needed for the new picker:
the thumbnails under 'My Photos' section are real-time representation
of the files under Downloads. We do not want to cache or save them
since the files may change all the time.

Therefore, to simplify the logic, we only need to read the image data,
and then use CSS to resize the image to display as the thumbnail
(the same with backdrop wallpapers for consistency).

The new wallpaper picker doesn't display the thumbnail of the history
custom wallpaper like the old picker. So basically the 'generate and
save thumbnail' code path can be deprecated in the future.

Bug: 809793
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ibf00bec636a52081c77cd7cf1b94e934fd97b11c
Reviewed-on: https://chromium-review.googlesource.com/912282Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536950}