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LogoutButtonTray uses and owns LogoutConfirmationDialogView, a dialog that
asks the user to confirm or deny logout. This CL generalizes the dialog
and moves its ownership to a LogoutConfirmationController accessible via
ash::Shell, allowing other classes to show the same dialog.

BUG=278247
TEST=Updated unit tests

Review URL: https://codereview.chromium.org/178183005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255316 0039d316-1c4b-4281-b951-d872f2087c98