VisibilityChecker: Make topoSort member final

Spotted by automated checkers.

Change-Id: I226a0a3efb8f527e5bde122e3f553039218d3418

java/com/google/gitiles/VisibilityChecker.java

diff --git a/java/com/google/gitiles/VisibilityChecker.java b/java/com/google/gitiles/VisibilityChecker.java
index e98377d..72aab3d 100644
--- a/java/com/google/gitiles/VisibilityChecker.java
+++ b/java/com/google/gitiles/VisibilityChecker.java
@@ -60,7 +60,7 @@
  */
 public class VisibilityChecker {
 
-  private boolean topoSort;
+  private final boolean topoSort;
 
   /**
    * @param topoSort whether to use a more thorough reachability check by sorting in topological