[Prerender] first contentful paint histograms.

chrome/browser/prerender/prerender_tab_helper.h

Index: chrome/browser/prerender/prerender_tab_helper.h
diff --git a/chrome/browser/prerender/prerender_tab_helper.h b/chrome/browser/prerender/prerender_tab_helper.h
index 19fb16e38a7dfd4e3bc44e61d2460016c60fcf2e..96e999531d399348aa165a40af307cbace49a462 100644
--- a/chrome/browser/prerender/prerender_tab_helper.h
+++ b/chrome/browser/prerender/prerender_tab_helper.h
@@ -51,6 +51,10 @@ class PrerenderTabHelper
   // Called when a control prerender is resolved. Applies to the next load.
   void WouldHavePrerenderedNextLoad(Origin origin);
 
+  base::TimeTicks last_swap() const { return last_swap_; }
+
+  Origin origin() const { return origin_; }
+
  private:
   explicit PrerenderTabHelper(content::WebContents* web_contents);
   friend class content::WebContentsUserData<PrerenderTabHelper>;
@@ -96,6 +100,10 @@ class PrerenderTabHelper
   // swapped in).
   base::TimeTicks actual_load_start_;
 
+  // Record the most recent swap time. This differs from |pplt_load_start_| in
+  // that it is not reset in various circumstances, like a load being stopped.
+  base::TimeTicks last_swap_;

[pasko, 2016/12/21 18:39:09]

My reading of the code suggests that:

1. Instances of this class do not travel from one WebContents to another, and
have only one instance per WebContents.

2. PrerenderTabHelper::PrerenderSwappedIn() gets recorded on the tab helper for
the _new_ WebContents.

Hence, a given PrerenderTabHelper can observe a swap only once. This makes
last_swap_ is confusing and is overly alarming. How about swap_time_ticks_?

[mattcary, 2016/12/22 10:49:07]

"Alarming" seems a bit of a stretch, but sure.

+
   // Current URL being loaded.
   GURL url_;