[NoStatePrefetch] Add UMA histogram to count prefetch requests

chrome/browser/prerender/prerender_resource_throttle.h

Index: chrome/browser/prerender/prerender_resource_throttle.h
diff --git a/chrome/browser/prerender/prerender_resource_throttle.h b/chrome/browser/prerender/prerender_resource_throttle.h
index f8a2c25501933b820a027707d5eed88f90a1003c..731ab3e906900a30b875c1966e433d19da316bed 100644
--- a/chrome/browser/prerender/prerender_resource_throttle.h
+++ b/chrome/browser/prerender/prerender_resource_throttle.h
@@ -37,6 +37,7 @@ class PrerenderResourceThrottle
   void WillStartRequest(bool* defer) override;
   void WillRedirectRequest(const net::RedirectInfo& redirect_info,
                            bool* defer) override;
+  void WillProcessResponse(bool* defer) override;
   const char* GetNameForLogging() const override;
 
   // Called by the PrerenderContents when a prerender becomes visible.
@@ -67,6 +68,11 @@ class PrerenderResourceThrottle
       int render_frame_id,
       const GURL& new_url);
 
+  static void UpdatePrefetchHistogramOnUI(bool is_main_resource,
+                                          bool is_main_frame,
+                                          int render_process_id,
+                                          int render_frame_id);
+
   // Helper to return the PrerenderContents given a render frame id. May return
   // NULL if it's gone.
   static PrerenderContents* PrerenderContentsFromRenderFrame(