Add support for changing roots through the root visitor callback.

Needed for copying collectors.

Change-Id: Icc4a342a57e0cfb79587edb02ef8c85e08808877
diff --git a/runtime/class_linker_test.cc b/runtime/class_linker_test.cc
index 6442f5a..192111f 100644
--- a/runtime/class_linker_test.cc
+++ b/runtime/class_linker_test.cc
@@ -340,8 +340,9 @@
     }
   }
 
-  static void TestRootVisitor(const mirror::Object* root, void*) {
+  static mirror::Object* TestRootVisitor(mirror::Object* root, void*) {
     EXPECT_TRUE(root != NULL);
+    return root;
   }
 };