ash: Fix use of uninitialized member in initializer list

A recent version of Clang warned about this:

  ../../ash/system/user/tray_user.cc:1055:20: error: field 'listener_' is uninitialized when used here [-Werror,-Wuninitialized]
      : CustomButton(listener_),
                     ^
BUG=323668
R=jennyz@chromium.org

Review URL: https://codereview.chromium.org/133743004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244277 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244277