[chromeos] Fix a use-after-free problem with LoginEventObserver

Always use the static function ExistingUserController::current_controller() to obtain the pointer to
the current user controller object. This is to prevent heap corruption when LoginEventObserver::Observe()
is called after the controller is already freed.

BUG=150568
TEST=tryserver

Review URL: https://codereview.chromium.org/11088032

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@161049 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed