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*) When adding new user, show the default wallpaper.
*) When clicking on a user, who requires online auth, load their wallpaper.
*) When cancelling online auth, revert to the last active user's wallpaper.

BUG=153700,139074


Review URL: https://chromiumcodereview.appspot.com/11190022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@162454 0039d316-1c4b-4281-b951-d872f2087c98