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/mirror/class.cc b/runtime/mirror/class.cc
index 6446d02..3208de9 100644
--- a/runtime/mirror/class.cc
+++ b/runtime/mirror/class.cc
@@ -52,8 +52,7 @@
 
 void Class::VisitRoots(RootCallback* callback, void* arg) {
   if (java_lang_Class_ != nullptr) {
-    java_lang_Class_ = down_cast<Class*>(
-        callback(java_lang_Class_, arg, 0, kRootStickyClass));
+    callback(reinterpret_cast<mirror::Object**>(&java_lang_Class_), arg, 0, kRootStickyClass);
   }
 }