Fix valgrind tests

Delete large objects in space destructor. Also some cleanup.

Change-Id: I4c4e90149841a156b7a3236201b37683e14890fb
diff --git a/runtime/indirect_reference_table.cc b/runtime/indirect_reference_table.cc
index 5012965..d6f9682 100644
--- a/runtime/indirect_reference_table.cc
+++ b/runtime/indirect_reference_table.cc
@@ -258,7 +258,10 @@
 void IndirectReferenceTable::VisitRoots(RootVisitor* visitor, const RootInfo& root_info) {
   BufferedRootVisitor<kDefaultBufferedRootCount> root_visitor(visitor, root_info);
   for (auto ref : *this) {
-    root_visitor.VisitRootIfNonNull(*ref);
+    if (!ref->IsNull()) {
+      root_visitor.VisitRoot(*ref);
+      DCHECK(!ref->IsNull());
+    }
   }
 }