The hidden attribute was erroneously left on the cancel button in diff
comments, causing it to not show up by default.
It showed up upon editing an existing draft because the hideOnPublished
CSS overrode the hack for the [hidden] attribute.
In addition, the buggy behavior was actually enforced by a buggy test --
it asserted that the button was not visible, but was labelled as "Cancel
is visible".
Bug: Issue 7664
Change-Id: I9e08c33fc4adf0f1465a52055a3308210625ffde
060b612810