Fix CC root visiting bug

Also some cleanup.

Change-Id: Ia3de8f2d409770be3619ec116e8b06ecd82338fe
diff --git a/runtime/reference_table.cc b/runtime/reference_table.cc
index ac36447..beba64f 100644
--- a/runtime/reference_table.cc
+++ b/runtime/reference_table.cc
@@ -238,7 +238,7 @@
 }
 
 void ReferenceTable::VisitRoots(RootVisitor* visitor, const RootInfo& root_info) {
-  BufferedRootVisitor<128> buffered_visitor(visitor, root_info);
+  BufferedRootVisitor<kDefaultBufferedRootCount> buffered_visitor(visitor, root_info);
   for (GcRoot<mirror::Object>& root : entries_) {
     buffered_visitor.VisitRoot(root);
   }