[heap] Move {hidden_string} into the root list.

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index 2c3d687786fe6d2b53c1ecbbe6baf3ce9c3add20..0828e81db5177e02b909eaf3c40e1e2cde36acfe 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -625,10 +625,6 @@ class Factory final {
     isolate()->heap()->set_weak_stack_trace_list(*list);
   }
 
-  Handle<String> hidden_string() {
-    return Handle<String>(&isolate()->heap()->hidden_string_);
-  }
-
   // Allocates a new SharedFunctionInfo object.
   Handle<SharedFunctionInfo> NewSharedFunctionInfo(
       Handle<String> name, int number_of_literals, FunctionKind kind,