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

chrome/browser/prerender/prerender_manager.cc

Index: chrome/browser/prerender/prerender_manager.cc
diff --git a/chrome/browser/prerender/prerender_manager.cc b/chrome/browser/prerender/prerender_manager.cc
index 323d20248f7bb7a27e54b42743942a3f2fc31add..b4f72250010d67047c7374439ac3c5069069c00d 100644
--- a/chrome/browser/prerender/prerender_manager.cc
+++ b/chrome/browser/prerender/prerender_manager.cc
@@ -447,6 +447,8 @@ bool PrerenderManager::AddPrerender(
   if (!prerender_contents || !prerender_contents->Init())
     return false;
 
+  histograms_->RecordPrerenderStarted(origin);
+
   // TODO(cbentzel): Move invalid checks here instead of PrerenderContents?
   PrerenderContentsData data(prerender_contents, GetCurrentTime());