Change root visitor to use Object**.

Simplifies code and improves the performance of root visiting since
we usually don't need to check to see if the object moved.

Change-Id: Iba998f5a15ae1fa1b53ca5226dd2168a411196cf
diff --git a/runtime/reference_table.cc b/runtime/reference_table.cc
index a9b17e0..f43a15b 100644
--- a/runtime/reference_table.cc
+++ b/runtime/reference_table.cc
@@ -234,7 +234,7 @@
 void ReferenceTable::VisitRoots(RootCallback* visitor, void* arg, uint32_t tid,
                                 RootType root_type) {
   for (auto& ref : entries_) {
-    ref = visitor(ref, arg, tid, root_type);
+    visitor(&ref, arg, tid, root_type);
   }
 }