Fix broken IdGeneratorTest

Change-Id: I6d83c3a4248344a25f2d188cf17da8db0efa37d2
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/util/IdGenerator.java b/gerrit-server/src/main/java/com/google/gerrit/server/util/IdGenerator.java
index 19c820e..78eb657 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/util/IdGenerator.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/util/IdGenerator.java
@@ -43,11 +43,15 @@
 
   /** Produce the next identifier. */
   public int next() {
-    return mix(salt, gen.getAndIncrement());
+    return mix(gen.getAndIncrement());
   }
 
   private static final int salt = 0x9e3779b9;
 
+  static int mix(int in) {
+    return mix(salt, in);
+  }
+
   /** A very simple bit permutation to mask a simple incrementer. */
   public static int mix(final int salt, final int in) {
     short v0 = hi16(in);