Background:
There're two ways for OEM's to specify customized default wallpapers:
1) Configured by cros-config, which results in the several switches. ([1])

2) From an online customization manifest. Currently it's only used by Basking device. ([2], [3])

Goal:
Investigate why there were two code paths in the first place (e.g. security implication related to OEM partition).

If possible, combine these into one. We should use an offline manifest instead of using code path 2) as is, because we must see the customized default wallpaper during OOBE without network, but 2) requires downloading the wallpaper first.

[1] https://chromium.googlesource.com/chromiumos/platform2/+/master/login_manager/chrome_setup.cc

[2]
https://cs.chromium.org/chromium/src/chrome/browser/chromeos/customization/customization_wallpaper_downloader.h?dr=CSs

[3]
https://cs.corp.google.com/piper///depot/google3/googledata/html/external_content/gstatic/chrome/chromeos-customization/oem2-basking.json?q=wallpaper+file:%5E//depot/google3/googledata/html/external_content/gstatic/chrome/chromeos-customization/+package:%5Epiper$&dr=C&l=3