Add histograms for session startup enum and number of URLs to open.

chrome/browser/prefs/session_startup_pref.h

Index: chrome/browser/prefs/session_startup_pref.h
diff --git a/chrome/browser/prefs/session_startup_pref.h b/chrome/browser/prefs/session_startup_pref.h
index 75ea9d3027ab22b7e77087207ec572a615dcf72d..1ac4e548ec038846c95aa703b5a6677ef9abf089 100644
--- a/chrome/browser/prefs/session_startup_pref.h
+++ b/chrome/browser/prefs/session_startup_pref.h
@@ -39,10 +39,13 @@ struct SessionStartupPref {
 
   // For historical reasons the enum and value registered in the prefs don't
   // line up. These are the values registered in prefs.
+  // The values are also recorded in Settings.StartupPageLoadSettings histogram,
+  // so make sure to update histograms.xml if you change these.
   static const int kPrefValueHomePage = 0;  // Deprecated
   static const int kPrefValueLast = 1;
   static const int kPrefValueURLs = 4;
   static const int kPrefValueNewTab = 5;
+  static const int kPrefValueMax = 6;
 
   static void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);