Simplifed logic of showing/hiding cancel buttons on login screens.

Screens doesn't rely on HeaderBar to make decisions about showing cancel buttons
anymore. Also removed 'cancel add users' header bar button, as it is not used
in the new UI.

BUG=484514

Review URL: https://codereview.chromium.org/1411423006

Cr-Commit-Position: refs/heads/master@{#355964}
9 files changed