Add histogram to track prerender sessions

chrome/browser/browser_main.h

Index: chrome/browser/browser_main.h
diff --git a/chrome/browser/browser_main.h b/chrome/browser/browser_main.h
index a8f544295e1c9205e96b893ee0175ff2698b215b..1c0b3711cd597d327a9167528b9cce41b6666870 100644
--- a/chrome/browser/browser_main.h
+++ b/chrome/browser/browser_main.h
@@ -118,8 +118,9 @@ class BrowserMainParts {
   // A/B test for spdy when --use-spdy not set.
   void SpdyFieldTrial();
 
-  // A/B test for prefetching with --(enable|disable)-prefetch not set.
-  void PrefetchFieldTrial();
+  // A/B test for prefetching with --(enable|disable)-prefetch not set,
+  // some histograms tracking prerender sessions.
+  void PrefetchAndPrerenderFieldTrial();
 
   // A/B test for automatically establishing a backup TCP connection when a
   // specified timeout value is reached.