[NoStatePrefetch] Add performance histograms.

chrome/browser/prerender/prerender_manager.h

Index: chrome/browser/prerender/prerender_manager.h
diff --git a/chrome/browser/prerender/prerender_manager.h b/chrome/browser/prerender/prerender_manager.h
index 83bd516b64b73a12326a400123cb58f121c3de61..11ca3e0d34bead03e29499a37c6a748a14048cd7 100644
--- a/chrome/browser/prerender/prerender_manager.h
+++ b/chrome/browser/prerender/prerender_manager.h
@@ -216,6 +216,13 @@ class PrerenderManager : public base::SupportsWeakPtr<PrerenderManager>,
                                    bool is_main_resource,
                                    int redirect_count);
 
+  // Records the time to first contentful paint.
+  // |is_no_store| must be true if the main resource has a "no-store" cache
+  // control HTTP header.
+  void RecordFirstContentfulPaint(const GURL& url,
+                                  bool is_no_store,
+                                  base::TimeDelta time);

[mmenke, 2016/09/16 16:53:48]

Oh, this does something completely different from what I assumed it did.  Could
we make it clearer that this is for recording the time of a non-prefetched page,
after it was previously prefetched, and rename the method as well to make that
clearer?

[droger, 2016/09/19 09:55:23]

Note that prefetched pages are not rendered at all, so the "first contentful
paint" does not mean anything for them.

Added a comment.

+
   static PrerenderManagerMode GetMode();
   static void SetMode(PrerenderManagerMode mode);
   static bool IsPrerenderingPossible();
@@ -501,7 +508,8 @@ class PrerenderManager : public base::SupportsWeakPtr<PrerenderManager>,
   void DeleteOldWebContents();
 
   // Cleans up old NavigationRecord's.
-  void CleanUpOldNavigations();
+  void CleanUpOldNavigations(std::vector<NavigationRecord>* navigations,
+                             base::TimeDelta max_age);
 
   // Arrange for the given WebContents to be deleted asap. Delete |deleter| as
   // well.
@@ -551,6 +559,9 @@ class PrerenderManager : public base::SupportsWeakPtr<PrerenderManager>,
   // |navigate_time_|.
   std::vector<NavigationRecord> navigations_;
 
+  // List of recent prefetches, sorted by ascending navigate time.
+  std::vector<NavigationRecord> prefetches_;
+
   std::unique_ptr<PrerenderContents::Factory> prerender_contents_factory_;
 
   static PrerenderManagerMode mode_;