Hoist getUserName up onto CurrentUser

Since all users have a unique username on the server, this makes
it cleaner to determine who is who.

Change-Id: Ifc97f3c3a5b469182beb95fdc2689e226cc38181
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/CurrentUser.java b/gerrit-server/src/main/java/com/google/gerrit/server/CurrentUser.java
index 5a68212..b9404b0 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/CurrentUser.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/CurrentUser.java
@@ -68,6 +68,11 @@
   /** Filters selecting changes the user wants to monitor. */
   public abstract Collection<AccountProjectWatch> getNotificationFilters();
 
+  /** Unique name of the user on this server, if one has been assigned. */
+  public String getUserName() {
+    return null;
+  }
+
   /** Capabilities available to this user account. */
   public CapabilityControl getCapabilities() {
     CapabilityControl ctl = capabilities;
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/IdentifiedUser.java b/gerrit-server/src/main/java/com/google/gerrit/server/IdentifiedUser.java
index 7c0e51e..722e3d7 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/IdentifiedUser.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/IdentifiedUser.java
@@ -228,6 +228,7 @@
   }
 
   /** @return the user's user name; null if one has not been selected/assigned. */
+  @Override
   public String getUserName() {
     return state().getUserName();
   }