active_user_ is reset to NULL when corresponding User object is deleted.

BUG=348235

Review URL: https://codereview.chromium.org/190743003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256220 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256220