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/stack_trace_element.cc b/runtime/mirror/stack_trace_element.cc
index 6bc695d..02a396a 100644
--- a/runtime/mirror/stack_trace_element.cc
+++ b/runtime/mirror/stack_trace_element.cc
@@ -70,8 +70,8 @@
 
 void StackTraceElement::VisitRoots(RootCallback* callback, void* arg) {
   if (java_lang_StackTraceElement_ != nullptr) {
-    java_lang_StackTraceElement_ = down_cast<Class*>(
-        callback(java_lang_StackTraceElement_, arg, 0, kRootStickyClass));
+    callback(reinterpret_cast<mirror::Object**>(&java_lang_StackTraceElement_), arg, 0,
+             kRootStickyClass);
   }
 }