Decouple root visitors from object visitors.

src/visitors.cc

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/visitors.h"
+
+namespace v8 {
+namespace internal {
+
+#define DECLARE_TAG(ignore1, name, ignore2) name,
+const char* const
+    VisitorSynchronization::kTags[VisitorSynchronization::kNumberOfSyncTags] = {
+        ROOT_ID_LIST(DECLARE_TAG)};
+#undef DECLARE_TAG
+
+#define DECLARE_TAG(ignore1, ignore2, name) name,
+const char* const VisitorSynchronization::kTagNames
+    [VisitorSynchronization::kNumberOfSyncTags] = {ROOT_ID_LIST(DECLARE_TAG)};
+#undef DECLARE_TAG
+
+}  // namespace internal
+}  // namespace v8