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

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

Index: chrome/browser/ui/webui/ntp/new_tab_page_handler.cc
diff --git a/chrome/browser/ui/webui/ntp/new_tab_page_handler.cc b/chrome/browser/ui/webui/ntp/new_tab_page_handler.cc
index d40e5d809dcd1981d4f7ab2758dad4af052a70d3..d8741ff5aea8563d66134eebdff280a70483e3b1 100644
--- a/chrome/browser/ui/webui/ntp/new_tab_page_handler.cc
+++ b/chrome/browser/ui/webui/ntp/new_tab_page_handler.cc
@@ -90,6 +90,11 @@ void NewTabPageHandler::HandlePageSelected(const ListValue* args) {
   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;