Always create FrameTreeNodes and RenderFrameHosts for every frame.

chrome/renderer/chrome_render_frame_observer.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 "chrome/renderer/chrome_render_frame_observer.h"
 
 #include "chrome/common/prerender_messages.h"
 #include "chrome/renderer/prerender/prerender_helper.h"
 
 ChromeRenderFrameObserver::ChromeRenderFrameObserver(
     content::RenderFrame* render_frame)
     : content::RenderFrameObserver(render_frame) {
 }
 
 ChromeRenderFrameObserver::~ChromeRenderFrameObserver() {
 }
 
 bool ChromeRenderFrameObserver::OnMessageReceived(const IPC::Message& message) {
   // Filter only.
   IPC_BEGIN_MESSAGE_MAP(ChromeRenderFrameObserver, message)
     IPC_MESSAGE_HANDLER(PrerenderMsg_SetIsPrerendering, OnSetIsPrerendering);
   IPC_END_MESSAGE_MAP()
 
   return false;
 }
 
 void ChromeRenderFrameObserver::OnSetIsPrerendering(bool is_prerendering) {
   if (is_prerendering) {
-    DCHECK(!prerender::PrerenderHelper::Get(render_frame()));
+    // If the PrerenderHelper for this frame already exists, don't create it. It
+    // can already be created for subframes during handling of
+    // RenderFrameCreated, if the parent frame was prerendering at time of
+    // subframe creation.
+    if (prerender::PrerenderHelper::Get(render_frame()))
+      return;
+
     // The PrerenderHelper will destroy itself either after recording histograms
     // or on destruction of the RenderView.
     new prerender::PrerenderHelper(render_frame());
   }
 }