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

chrome/browser/prerender/prerender_histograms.cc

Index: chrome/browser/prerender/prerender_histograms.cc
diff --git a/chrome/browser/prerender/prerender_histograms.cc b/chrome/browser/prerender/prerender_histograms.cc
index 9e772f938972b1f10f49c5e799cec6759260e183..1eaa5e469a1059aeb1989a5706d839bf1d24932b 100644
--- a/chrome/browser/prerender/prerender_histograms.cc
+++ b/chrome/browser/prerender/prerender_histograms.cc
@@ -153,7 +153,9 @@ void PrerenderHistograms::RecordPrerender(Origin origin, const GURL& url) {
   last_prerender_seen_time_ = GetCurrentTimeTicks();
   seen_any_pageload_ = false;
   seen_pageload_started_after_prerender_ = false;
+}
 
+void PrerenderHistograms::RecordPrerenderStarted(Origin origin) const {
   if (OriginIsOmnibox(origin)) {
     UMA_HISTOGRAM_COUNTS("Prerender.OmniboxPrerenderCount_" +
                          GetOmniboxHistogramSuffix(), 1);