Mark graph visitor classes as final.

This should improve optimization opportunities for clang++.

Test: buildbot-build.sh
Change-Id: Ib0c1ebfb157176a9063cca2ed465cff6fb280442
diff --git a/compiler/optimizing/write_barrier_elimination.cc b/compiler/optimizing/write_barrier_elimination.cc
index 9023cc2..eb70b67 100644
--- a/compiler/optimizing/write_barrier_elimination.cc
+++ b/compiler/optimizing/write_barrier_elimination.cc
@@ -23,7 +23,7 @@
 
 namespace art HIDDEN {
 
-class WBEVisitor : public HGraphVisitor {
+class WBEVisitor final : public HGraphVisitor {
  public:
   WBEVisitor(HGraph* graph, OptimizingCompilerStats* stats)
       : HGraphVisitor(graph),