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

chrome/browser/android/offline_pages/prerendering_loader.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/prerendering_loader.h"
 
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/threading/thread_task_runner_handle.h"
@@ skipping 40 matching lines @@
   bool accepted = adapter_->StartPrerender(
       browser_context_, url, sessionStorageNamespace, renderWindowSize);
   if (!accepted)
     return false;
 
   DCHECK(adapter_->IsActive());
   snapshot_controller_.reset(
       new SnapshotController(base::ThreadTaskRunnerHandle::Get(), this));
   callback_ = callback;
   session_contents_.swap(new_web_contents);
   state_ = State::PENDING;

[dougarnett, 2016/09/21 16:16:51]

Could just set LOADING here instead and drop the OnPrerenderStart logic and
maybe method.

[dougarnett, 2016/09/21 16:32:40]

BTW, I could do that clean up as it is a bit more involved. 

[qinmin, 2016/09/21 16:40:04]

Can you take another look at the new patch, I have cleaned up the
OnPrerenderStart() method

+  // Note: Observer is attached too late when calling
+  // PrerenderAdapter::StartPrerenderer(). Call the missing OnPrerenderStart()
+  // here.
+  OnPrerenderStart();
   return true;
 }
 
 void PrerenderingLoader::StopLoading() {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   CancelPrerender();
 }
 
 bool PrerenderingLoader::CanPrerender() {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
@@ skipping 127 matching lines @@
 void PrerenderingLoader::CancelPrerender() {
   if (adapter_->IsActive()) {
     adapter_->DestroyActive();
   }
   snapshot_controller_.reset(nullptr);
   session_contents_.reset(nullptr);
   state_ = State::IDLE;
 }
 
 }  // namespace offline_pages