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/string.cc b/runtime/mirror/string.cc
index cd63c39..3f35210 100644
--- a/runtime/mirror/string.cc
+++ b/runtime/mirror/string.cc
@@ -288,7 +288,7 @@
 
 void String::VisitRoots(RootCallback* callback, void* arg) {
   if (java_lang_String_ != nullptr) {
-    java_lang_String_ = down_cast<Class*>(callback(java_lang_String_, arg, 0, kRootStickyClass));
+    callback(reinterpret_cast<mirror::Object**>(&java_lang_String_), arg, 0, kRootStickyClass);
   }
 }