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/class_linker_test.cc b/runtime/class_linker_test.cc
index 28ed6c4..ebf02fe 100644
--- a/runtime/class_linker_test.cc
+++ b/runtime/class_linker_test.cc
@@ -339,9 +339,8 @@
     }
   }
 
-  static mirror::Object* TestRootVisitor(mirror::Object* root, void*, uint32_t, RootType) {
-    EXPECT_TRUE(root != NULL);
-    return root;
+  static void TestRootVisitor(mirror::Object** root, void*, uint32_t, RootType) {
+    EXPECT_TRUE(*root != NULL);
   }
 };