[NoStatePrefetch] Add UMA histogram to count prefetch requests

chrome/browser/prerender/prerender_histograms.h

Index: chrome/browser/prerender/prerender_histograms.h
diff --git a/chrome/browser/prerender/prerender_histograms.h b/chrome/browser/prerender/prerender_histograms.h
index f48c850b7569d0b989d849fd894f153c621d567f..f2d5de391b00176fbe5777c90cbbee2ebd41bdd4 100644
--- a/chrome/browser/prerender/prerender_histograms.h
+++ b/chrome/browser/prerender/prerender_histograms.h
@@ -11,6 +11,7 @@
 #include <string>
 
 #include "base/macros.h"
+#include "base/threading/thread_checker.h"
 #include "base/time/time.h"
 #include "chrome/browser/prerender/prerender_contents.h"
 #include "chrome/browser/prerender/prerender_final_status.h"
@@ -105,7 +106,12 @@ class PrerenderHistograms {
   void RecordNetworkBytes(Origin origin,
                           bool used,
                           int64_t prerender_bytes,
-                          int64_t profile_bytes);
+                          int64_t profile_bytes) const;
+
+  // Record resources loaded by NoStatePrefetch.
+  void RecordResourcePrefetch(Origin origin,
+                              bool is_main_resource,
+                              bool is_no_store) const;
 
  private:
   base::TimeTicks GetCurrentTimeTicks() const;
@@ -139,6 +145,8 @@ class PrerenderHistograms {
   bool seen_any_pageload_;
   bool seen_pageload_started_after_prerender_;
 
+  base::ThreadChecker thread_checker_;
+
   DISALLOW_COPY_AND_ASSIGN(PrerenderHistograms);
 };