Fix wallpaper revert to the first user's wallpaper after focus on the second user at login screen


BUG=350079

Review URL: https://codereview.chromium.org/197343004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256718 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256718