Clarify comment on FrameTree::AddFrame.

content/browser/frame_host/frame_tree.h

 // 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.
 
 #ifndef CONTENT_BROWSER_FRAME_HOST_FRAME_TREE_H_
 #define CONTENT_BROWSER_FRAME_HOST_FRAME_TREE_H_
 
 #include <string>
 
 #include "base/callback.h"
@@ skipping 52 matching lines @@
   FrameTreeNode* FindByRoutingID(int routing_id, int process_id);
 
   // Executes |on_node| on each node in the frame tree.  If |on_node| returns
   // false, terminates the iteration immediately. Returning false is useful
   // if |on_node| is just doing a search over the tree.  The iteration proceeds
   // top-down and visits a node before adding its children to the queue, making
   // it safe to remove children during the callback.
   void ForEach(const base::Callback<bool(FrameTreeNode*)>& on_node) const;
 
   // Frame tree manipulation routines.
+  // |process_id| is required to disambiguate |new_routing_id|, and it must
+  // match the process of the |parent| node.  Otherwise this method returns
+  // nullptr.  Passing MSG_ROUTING_NONE for |new_routing_id| will allocate a new
+  // routing ID for the new frame.
   RenderFrameHostImpl* AddFrame(FrameTreeNode* parent,
                                 int process_id,
                                 int new_routing_id,
                                 const std::string& frame_name);
   void RemoveFrame(FrameTreeNode* child);
 
   // This method walks the entire frame tree and creates a RenderFrameProxyHost
   // for the given |site_instance| in each node except the |source| one --
   // the source will have a RenderFrameHost. It assumes that no frame tree
   // nodes already have RenderFrameProxyHost for the given |site_instance|.
@@ skipping 80 matching lines @@
   int64 focused_frame_tree_node_id_;
 
   base::Callback<void(RenderFrameHost*)> on_frame_removed_;
 
   DISALLOW_COPY_AND_ASSIGN(FrameTree);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_FRAME_HOST_FRAME_TREE_H_