Fix an issue that PrerenderLoader::OnPrerenderStart() never gets called.

chrome/browser/android/offline_pages/prerender_adapter.cc

 // Copyright 2016 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/browser/android/offline_pages/prerender_adapter.h"
 
 #include "chrome/browser/prerender/prerender_manager.h"
 #include "chrome/browser/prerender/prerender_manager_factory.h"
 #include "chrome/browser/profiles/profile.h"
 #include "content/public/browser/browser_context.h"
@@ skipping 26 matching lines @@
     const gfx::Size& size) {
   DCHECK(!IsActive());
   DCHECK(CanPrerender());
 
   Profile* profile = Profile::FromBrowserContext(browser_context);
   prerender::PrerenderManager* manager =
       prerender::PrerenderManagerFactory::GetForProfile(profile);
   DCHECK(manager);
 
   // Start prerendering the url and capture the handle for the prerendering.
+  // Note: PrerenderHandle::OnPrerenderStart() will get called inside
+  // PrerenderManager::AddPrerenderForOffline(), this is before the observer is
+  // set.
   active_handle_ =
       manager->AddPrerenderForOffline(url, session_storage_namespace, size);
   if (!active_handle_)
     return false;
+  DCHECK(active_handle_->contents());
+  DCHECK(active_handle_->contents()->prerendering_has_started());
 
   active_handle_->SetObserver(this);
+
   return true;
 }
 
 content::WebContents* PrerenderAdapter::GetWebContents() const {
   DCHECK(IsActive());
   DCHECK(active_handle_->contents());
   // Note: the prerender stack maintains ownership of these contents
   // and PrerenderingLoader::StopLoading() must be called to report
   // the Loader is done with the contents.
   return active_handle_->contents()->prerender_contents();
@@ skipping 36 matching lines @@
 }
 
 void PrerenderAdapter::OnPrerenderStop(prerender::PrerenderHandle* handle) {
   if (IsActive()) {
     DCHECK_EQ(active_handle_.get(), handle);
     observer_->OnPrerenderStop();
   }
 }
 
 }  // namespace offline_pages