ash: Fix DesktopBackgroundController getting stuck.

Fix an issue where DesktopBackgroundController gets confused
and stops switching wallpapers. The following sequence of
events triggers the bug:

a) Default wallpaper is requested and starts getting loaded
   from disk.
b) Before default wallpaper has been loaded, custom
   wallpaper is requested.
c) Default wallpaper is requested again.

At b), the default wallpaper load operation is canceled but
the WallpaperLoader object is retained. At c),
DesktopBackgroundController incorrectly sees the
still-present WallpaperLoader and believes that the default
wallpaper is still being loaded. This leaves
DesktopBackgroundController in a state where it never
switches back to the default wallpaper.

The fix is to disregard the existing WallpaperLoader if it
has already been canceled.

BUG=327443

Review URL: https://codereview.chromium.org/110463004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242579 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242579