Merge 159822 - ash: Display system background while loading wallpaper.

Merge 159822 - ash: Display system background while loading wallpaper.

When the wallpaper takes a long time to load, areas of the
screen that aren't covered by other layers are painted
white.  This looks okay when we first boot, but not when
we're displaying the login after logging out or while we're
starting a guest session.

This makes us instead initialize SystemBackgroundController
immediately with either a #fefefe or black color depending
on whether this is the first Chrome run after boot or not.
Its color is updated to black after the login wallpaper
animation is finished.

BUG=151111, 152751
Review URL: https://chromiumcodereview.appspot.com/11054005

TBR=derat@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=160654