Add recording of when a prerender actually starts to prerender histograms.

chrome/browser/prerender/prerender_histograms.h

 // Copyright (c) 2011 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_PRERENDER_PRERENDER_HISTOGRAMS_H_
 #define CHROME_BROWSER_PRERENDER_PRERENDER_HISTOGRAMS_H_
 #pragma once
 
 #include <string>
 
@@ skipping 37 matching lines @@
 
   // Record a final status of a prerendered page in a histogram.
   void RecordFinalStatus(Origin origin,
                          uint8 experiment_id,
                          FinalStatus final_status) const;
 
 
   // To be called when a new prerender is added.
   void RecordPrerender(Origin origin, const GURL& url);
 
+  // To be called when a new prerender is started.
+  void RecordPrerenderStarted(Origin origin) const;
+
   // Called when we swap in a prerender.
   void RecordUsedPrerender(Origin origin) const;
 
  private:
   base::TimeTicks GetCurrentTimeTicks() const;
 
   // Returns the time elapsed since the last prerender happened.
   base::TimeDelta GetTimeSinceLastPrerender() const;
 
   // Returns whether the PrerenderManager is currently within the prerender
@@ skipping 30 matching lines @@
   // start recording events before the first prerender occurs.
   bool seen_any_pageload_;
   bool seen_pageload_started_after_prerender_;
 
   DISALLOW_COPY_AND_ASSIGN(PrerenderHistograms);
 };
 
 }  // namespace prerender
 
 #endif  // CHROME_BROWSER_PRERENDER_PRERENDER_HISTOGRAMS_H_