Fix crash on wallpaperPrivate api with multi profile turned on

This fixes the crash described in 309968. But there are more
cases that this API may fail. See Issue 322687. Added a todo
in this CL to address it.

BUG=309968

Review URL: https://codereview.chromium.org/78663005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237955 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237955