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/thread_list.cc b/runtime/thread_list.cc
index 25f692d..f058dee 100644
--- a/runtime/thread_list.cc
+++ b/runtime/thread_list.cc
@@ -771,11 +771,10 @@
   void* const arg_;
 };
 
-static mirror::Object* VerifyRootWrapperCallback(mirror::Object* root, void* arg,
-                                                 uint32_t /*thread_id*/, RootType /*root_type*/) {
+static void VerifyRootWrapperCallback(mirror::Object** root, void* arg, uint32_t /*thread_id*/,
+                                      RootType /*root_type*/) {
   VerifyRootWrapperArg* wrapperArg = reinterpret_cast<VerifyRootWrapperArg*>(arg);
-  wrapperArg->callback_(root, wrapperArg->arg_, 0, NULL);
-  return root;
+  wrapperArg->callback_(*root, wrapperArg->arg_, 0, NULL);
 }
 
 void ThreadList::VerifyRoots(VerifyRootCallback* callback, void* arg) const {