Main frame's unique name can always be null.

content/browser/frame_host/frame_tree_node.cc

Index: content/browser/frame_host/frame_tree_node.cc
diff --git a/content/browser/frame_host/frame_tree_node.cc b/content/browser/frame_host/frame_tree_node.cc
index 8af110a789dc77d5e28b7e38653f807a237a2dbb..9d6eac59956f040c18ba9478aa7efcc9d024b4c9 100644
--- a/content/browser/frame_host/frame_tree_node.cc
+++ b/content/browser/frame_host/frame_tree_node.cc
@@ -232,6 +232,15 @@ void FrameTreeNode::SetFrameName(const std::string& name,
     DCHECK_EQ(unique_name, replication_state_.unique_name);
     return;
   }
+
+  if (parent()) {
+    // Non-main frames should have a non-empty unique name.
+    DCHECK(!unique_name.empty());
+  } else {
+    // Unique name of main frames should always stay empty.
+    DCHECK(unique_name.empty());
+  }
+
   RecordUniqueNameLength(unique_name.size());
   render_manager_.OnDidUpdateName(name, unique_name);
   replication_state_.name = name;