Clarify comment on FrameTree::AddFrame.

content/browser/frame_host/frame_tree.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.h"
 
 #include <queue>
 
 #include "base/bind.h"
 #include "base/callback.h"
@@ skipping 132 matching lines @@
 }
 
 RenderFrameHostImpl* FrameTree::AddFrame(FrameTreeNode* parent,
                                          int process_id,
                                          int new_routing_id,
                                          const std::string& frame_name) {
   // A child frame always starts with an initial empty document, which means
   // it is in the same SiteInstance as the parent frame. Ensure that the process
   // which requested a child frame to be added is the same as the process of the
   // parent node.
+  // We return nullptr if this is not the case, which can happen in a race if an
+  // old RFH sends a CreateChildFrame message as we're swapping to a new RFH.
   if (parent->current_frame_host()->GetProcess()->GetID() != process_id)
     return nullptr;
 
   scoped_ptr<FrameTreeNode> node(new FrameTreeNode(
       this, parent->navigator(), render_frame_delegate_, render_view_delegate_,
       render_widget_delegate_, manager_delegate_, frame_name));
   std::pair<FrameTreeNodeIDMap::iterator, bool> result =
       g_frame_tree_node_id_map.Get().insert(
           std::make_pair(node->frame_tree_node_id(), node.get()));
   CHECK(result.second);
@@ skipping 162 matching lines @@
         rvh->Shutdown();
         render_view_host_pending_shutdown_map_.erase(multi_iter);
       }
       break;
     }
     CHECK(render_view_host_found);
   }
 }
 
 }  // namespace content