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1) The full-screen white background in OOBE is rendered in web-ui, so
   there's a delay showing it. Users will see a glimpse of the default
   wallpaper (it's blue on an eve) between the boot splash screen
   and the OOBE screen (both white). To avoid this, we can show a white
   wallpaper during the entire OOBE screen, which can show right after
   the boot splash seamlessly.

2) The original idea was to delay the dismissal of the boot splash
   screen until OOBE dialog is ready. However, boot splash screen will
   not always show (e.g. when a guest user exits, or browser crash and
   restart). In such cases, we also want to avoid showing the blue
   default wallpaper before the white OOBE screen is loaded. Therefore,
   showing a white wallpaper is a better solution.

Bug: 872936
Change-Id: Iaa600f9d455956743a6cfd3dd26f07ad2a4ed61d
Reviewed-on: https://chromium-review.googlesource.com/1172066Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Oliver Chang <ochang@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583154}