LUCENE-5892: CharsRefBuilder implements Accountable.


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1618627 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5892: Missed the covariant return type.


git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1618629 13f79535-47bb-0310-9956-ffa450edef68