FrameTree::Find only searches relative to local frames.

third_party/WebKit/Source/web/WebLocalFrameImpl.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 230 matching lines @@
   // WebLocalFrame methods:
   WebLocalFrameImpl* CreateLocalChild(WebTreeScopeType,
                                       WebFrameClient*,
                                       blink::InterfaceProvider*,
                                       blink::InterfaceRegistry*) override;
   void SetAutofillClient(WebAutofillClient*) override;
   WebAutofillClient* AutofillClient() override;
   void SetDevToolsAgentClient(WebDevToolsAgentClient*) override;
   WebDevToolsAgent* DevToolsAgent() override;
   WebLocalFrameImpl* LocalRoot() override;
+  WebFrame* FindFrameByName(const WebString& name) override;
   void SendPings(const WebURL& destination_url) override;
   bool DispatchBeforeUnloadEvent(bool) override;
   WebURLRequest RequestFromHistoryItem(const WebHistoryItem&,
                                        WebCachePolicy) const override;
   WebURLRequest RequestForReload(WebFrameLoadType,
                                  const WebURL&) const override;
   void Load(const WebURLRequest&,
             WebFrameLoadType,
             const WebHistoryItem&,
             WebHistoryLoadType,
@@ skipping 260 matching lines @@
 
 DEFINE_TYPE_CASTS(WebLocalFrameImpl,
                   WebFrame,
                   frame,
                   frame->IsWebLocalFrame(),
                   frame.IsWebLocalFrame());
 
 }  // namespace blink
 
 #endif