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

chrome/browser/android/offline_pages/prerender_adapter.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_PRERENDER_ADAPTER_H_
 #define CHROME_BROWSER_ANDROID_OFFLINE_PAGES_PRERENDER_ADAPTER_H_
 
 #include <memory>
 
 #include "chrome/browser/prerender/prerender_handle.h"
@@ skipping 12 matching lines @@
 
 namespace offline_pages {
 
 // An adapter for managing a prerendering request for offlining. This handles
 // all calls into the prerender stack and tracks the active prerender handle.
 class PrerenderAdapter : public prerender::PrerenderHandle::Observer {
  public:
   // An observer of PrerenderHandle events that does not expose the handle.
   class Observer {
    public:
-    // Signals that the prerender has started running.
-    virtual void OnPrerenderStart() = 0;
-
     // Signals that the prerender has had its load event.
     virtual void OnPrerenderStopLoading() = 0;
 
     // Signals that the prerender has had its 'DOMContentLoaded' event.
     virtual void OnPrerenderDomContentLoaded() = 0;
 
     // Signals that the prerender has stopped running and any retrieved
     // WebContents (via |GetWebContents()|) have become invalidated.
     virtual void OnPrerenderStop() = 0;
 
@@ skipping 51 matching lines @@
 
   // Observer of active handle events. Not owned.
   PrerenderAdapter::Observer* observer_;
 
   DISALLOW_COPY_AND_ASSIGN(PrerenderAdapter);
 };
 
 }  // namespace offline_pages
 
 #endif  // CHROME_BROWSER_ANDROID_OFFLINE_PAGES_PRERENDER_ADAPTER_H_