Extend AbstractGroupBackend to implement a new GroupBackend method

Change-Id: Ic376f129b2b21eb649055355b9ae0a573e0f22ee

src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java b/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java
index d05b10c..9d72b1b 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/singleusergroup/SingleUserGroup.java
@@ -29,6 +29,7 @@
 import com.google.gerrit.reviewdb.client.AccountGroup;
 import com.google.gerrit.reviewdb.server.ReviewDb;
 import com.google.gerrit.server.IdentifiedUser;
+import com.google.gerrit.server.account.AbstractGroupBackend;
 import com.google.gerrit.server.account.AccountCache;
 import com.google.gerrit.server.account.AccountControl;
 import com.google.gerrit.server.account.AccountState;
@@ -57,7 +58,7 @@
  * account. A user can only be used as a group if it has a username.
  */
 @Singleton
-public class SingleUserGroup implements GroupBackend {
+public class SingleUserGroup extends AbstractGroupBackend {
   private static final Logger log =
       LoggerFactory.getLogger(SingleUserGroup.class);