Add HeapTest and make GC work enough to pass it

Change-Id: If06eaef2921b64b3226bfd347acaec60ec993e67
diff --git a/src/runtime.cc b/src/runtime.cc
index 50f25fe..1ac45f4 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -383,4 +383,9 @@
   return true;
 }
 
+void Runtime::VisitRoots(Heap::RootVistor* root_visitor, void* arg) const {
+  GetClassLinker()->VisitRoots(root_visitor, arg);
+  UNIMPLEMENTED(WARNING) << "mark other roots including per thread state and thread stacks";
+}
+
 }  // namespace art