Clean up class visitors

Move from function pointers to virtual function visitors.

Change-Id: I68cb83c1d2ed9b5a89f8e534fe7ca4bbc1c91f45
diff --git a/runtime/class_table.cc b/runtime/class_table.cc
index f775235..c245d4e 100644
--- a/runtime/class_table.cc
+++ b/runtime/class_table.cc
@@ -71,10 +71,10 @@
   }
 }
 
-bool ClassTable::Visit(ClassVisitor* visitor, void* arg) {
+bool ClassTable::Visit(ClassVisitor* visitor) {
   for (ClassSet& class_set : classes_) {
     for (GcRoot<mirror::Class>& root : class_set) {
-      if (!visitor(root.Read(), arg)) {
+      if (!visitor->Visit(root.Read())) {
         return false;
       }
     }