Prerender: clarify how FCP works in non-prerendered cases

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 db276ab7cbceff968df195fc7b7405f3854fc5bd..48d092df35a2430bb64abb4324780e8574d1f7c9 100644
--- a/chrome/browser/prerender/prerender_manager.h
+++ b/chrome/browser/prerender/prerender_manager.h
@@ -198,10 +198,19 @@ class PrerenderManager : public content::NotificationObserver,
                                    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,