FrameTree::Find only searches relative to local frames.

third_party/WebKit/Source/web/WebLocalFrameImpl.cpp

 /*
  * 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 1998 matching lines @@
 WebLocalFrameImpl* WebLocalFrameImpl::LocalRoot() {
   // This can't use the LocalFrame::localFrameRoot, since it may be called
   // when the WebLocalFrame exists but the core LocalFrame does not.
   // TODO(alexmos, dcheng): Clean this up to only calculate this in one place.
   WebLocalFrameImpl* local_root = this;
   while (local_root->Parent() && local_root->Parent()->IsWebLocalFrame())
     local_root = ToWebLocalFrameImpl(local_root->Parent());
   return local_root;
 }
 
+WebFrame* WebLocalFrameImpl::FindFrameByName(const WebString& name) {
+  Frame* result = GetFrame()->Tree().Find(name);
+  return WebFrame::FromFrame(result);

[dcheng, 2017/05/26 18:28:05]

The original helper on WebView only returned local frames it looks like (and
returned nullptr otherwise). Do you think we should keep the same interface?

[Łukasz Anforowicz, 2017/05/26 20:04:05]

I think these 2 concerns (finding a frame by name in the browsing instance VS
figuring out if a frame is local) are separate and should not be shoehorned into
FindFrameByName (FindFrameByNameAndCheckIfResultIsALocalFrame?).  The callers
who need to check if the frame is local (only some of the tests AFAICT) can
always call result->IsWebLocalFrame() after calling FindFrameByName.

+}
+
 void WebLocalFrameImpl::SendPings(const WebURL& destination_url) {
   DCHECK(GetFrame());
   DCHECK(context_menu_node_.Get());
   Element* anchor = context_menu_node_->EnclosingLinkEventParentOrSelf();
   if (isHTMLAnchorElement(anchor))
     toHTMLAnchorElement(anchor)->SendPings(destination_url);
 }
 
 bool WebLocalFrameImpl::DispatchBeforeUnloadEvent(bool is_reload) {
   if (!GetFrame())
@@ skipping 553 matching lines @@
 TextCheckerClient& WebLocalFrameImpl::GetTextCheckerClient() const {
   return *text_checker_client_;
 }
 
 void WebLocalFrameImpl::SetTextCheckClient(
     WebTextCheckClient* text_check_client) {
   text_check_client_ = text_check_client;
 }
 
 }  // namespace blink