ntp: add more UMA to prove existing UMA is sane.

chrome/browser/ui/webui/ntp/new_tab_page_handler.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/webui/ntp/new_tab_page_handler.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/field_trial.h"
@@ skipping 72 matching lines @@
 void NewTabPageHandler::HandlePageSelected(const ListValue* args) {
   double page_id_double;
   CHECK(args->GetDouble(0, &page_id_double));
   int page_id = static_cast<int>(page_id_double);
 
   double index_double;
   CHECK(args->GetDouble(1, &index_double));
   int index = static_cast<int>(index_double);
 
   PrefService* prefs = Profile::FromWebUI(web_ui())->GetPrefs();
+  int previous_shown_page =
+      prefs->GetInteger(prefs::kNTPShownPage) >> kPageIdOffset;
+  UMA_HISTOGRAM_ENUMERATION("NewTabPage.PreviousSelectedPageType",
+                            previous_shown_page, kHistogramEnumerationMax);
+
   prefs->SetInteger(prefs::kNTPShownPage, page_id | index);
 
   int shown_page_type = page_id >> kPageIdOffset;
   UMA_HISTOGRAM_ENUMERATION("NewTabPage.SelectedPageType",
                             shown_page_type, kHistogramEnumerationMax);
 
   static bool default_apps_trial_exists =
       base::FieldTrialList::TrialExists(kDefaultAppsTrial_Name);
   if (default_apps_trial_exists) {
     UMA_HISTOGRAM_ENUMERATION(
@@ skipping 67 matching lines @@
   // is only called during startup before the ntp resource cache is constructed.
 }
 
 void NewTabPageHandler::Notify(chrome::NotificationType notification_type) {
   content::NotificationService* service =
       content::NotificationService::current();
   service->Notify(notification_type,
                   content::Source<NewTabPageHandler>(this),
                   content::NotificationService::NoDetails());
 }