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

chrome/browser/prerender/prerender_histograms.h

Index: chrome/browser/prerender/prerender_histograms.h
diff --git a/chrome/browser/prerender/prerender_histograms.h b/chrome/browser/prerender/prerender_histograms.h
index a4453a484f901d2e07b3c66424cface90390248f..69c61fe4f32f3b95e0d9fa75203ecc557d58b7a2 100644
--- a/chrome/browser/prerender/prerender_histograms.h
+++ b/chrome/browser/prerender/prerender_histograms.h
@@ -55,6 +55,9 @@ class PrerenderHistograms {
   // 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;