Main frame's unique name can always be null.

content/browser/frame_host/frame_tree_node.cc

 // Copyright 2013 The Chromium 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 "content/browser/frame_host/frame_tree_node.h"
 
 #include <queue>
 #include <utility>
 
 #include "base/macros.h"
@@ skipping 214 matching lines @@
       is_potentially_trustworthy_unique_origin;
 }
 
 void FrameTreeNode::SetFrameName(const std::string& name,
                                  const std::string& unique_name) {
   if (name == replication_state_.name) {
     // |unique_name| shouldn't change unless |name| changes.
     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;
   replication_state_.unique_name = unique_name;
 }
 
 void FrameTreeNode::AddContentSecurityPolicy(
     const ContentSecurityPolicyHeader& header) {
   replication_state_.accumulated_csp_headers.push_back(header);
   render_manager_.OnDidAddContentSecurityPolicy(header);
@@ skipping 249 matching lines @@
       }
       return parent_->child_at(i + relative_offset);
     }
   }
 
   NOTREACHED() << "FrameTreeNode not found in its parent's children.";
   return nullptr;
 }
 
 }  // namespace content