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

chrome/browser/android/offline_pages/prerendering_loader.h

 // 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.
 
 #ifndef CHROME_BROWSER_ANDROID_OFFLINE_PAGES_PRERENDERING_LOADER_H_
 #define CHROME_BROWSER_ANDROID_OFFLINE_PAGES_PRERENDERING_LOADER_H_
 
 #include <memory>
 
 #include "base/callback.h"
@@ skipping 58 matching lines @@
   // Returns whether the loader has successfully loaded web contents.
   // Note that |StopLoading()| should be used to clear this state once
   // the loaded web contents are no longer needed.
   virtual bool IsLoaded();
 
   // Overrides the prerender stack adapter for unit testing.
   void SetAdapterForTesting(
       std::unique_ptr<PrerenderAdapter> prerender_adapter);
 
   // PrerenderAdapter::Observer implementation:
-  void OnPrerenderStart() override;
   void OnPrerenderStopLoading() override;
   void OnPrerenderDomContentLoaded() override;
   void OnPrerenderStop() override;
 
   // SnapshotController::Client implementation:
   void StartSnapshot() override;
 
  private:
   // State of the loader (only one request may be active at a time).
   enum class State {
@@ skipping 35 matching lines @@
   // Callback to call when the active load request completes, fails, or is
   // canceled.
   LoadPageCallback callback_;
 
   DISALLOW_COPY_AND_ASSIGN(PrerenderingLoader);
 };
 
 }  // namespace offline_pages
 
 #endif  // CHROME_BROWSER_ANDROID_OFFLINE_PAGES_PRERENDERING_LOADER_H_