Prerender: clarify how FCP works in non-prerendered cases

chrome/browser/prerender/prerender_manager.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_PRERENDER_PRERENDER_MANAGER_H_
 #define CHROME_BROWSER_PRERENDER_PRERENDER_MANAGER_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 180 matching lines @@
                                       bool is_main_resource,
                                       bool is_redirect,
                                       bool is_no_store);
 
   // Called when a NoStatePrefetch resource has been loaded. This is called only
   // once per resource, when all redirects have been resolved.
   void RecordPrefetchRedirectCount(Origin origin,
                                    bool is_main_resource,
                                    int redirect_count);
 
-  // Records the time to first contentful paint for loads that previously had a
-  // no state prefetch load.  Must not be called for prefetch loads themselves
-  // (which are never rendered anyway).  |is_no_store| must be true if the main
-  // resource has a "no-store" cache control HTTP header.
+  // Called to record the time to First Contentful Paint for all pages that were
+  // not prerendered.
+  //
+  // As part of recording, determines whether the load had previously matched
+  // the criteria for triggering a NoStatePrefetch. In the prerendering
+  // experimental group such triggering makes the page prerendered, while in the
+  // group doing only 'simple loads' it would have been a noop.
+  //
+  // Must not be called for prefetch loads themselves (which are never painted
+  // anyway). The |is_no_store| must be true iff the main resource has a
+  // "no-store" cache control HTTP header. The |was_hidden| tells whether the
+  // the page was hidden at least once between starting the load and registering
+  // the FCP.
   void RecordNoStateFirstContentfulPaint(const GURL& url,
                                          bool is_no_store,
                                          bool was_hidden,
                                          base::TimeDelta time);
 
   // Records the perceived first contentful paint time for a prerendered page.
   // The actual load may have started prior to navigation due to prerender
   // hints. The FCP ticks is in absolute time; this has the disadvantage that
   // the histogram will mix browser and renderer ticks, but there seems to be no
   // way around that.
@@ skipping 415 matching lines @@
   std::vector<std::unique_ptr<PrerenderManagerObserver>> observers_;
 
   base::WeakPtrFactory<PrerenderManager> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PrerenderManager);
 };
 
 }  // namespace prerender
 
 #endif  // CHROME_BROWSER_PRERENDER_PRERENDER_MANAGER_H_