Prerender: clarify how FCP works in non-prerendered cases

chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h

Index: chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h
diff --git a/chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h b/chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h
index bcebe45350d7db4e8fccecd50f75c8c8ce2d463e..7e74ccff073730050965ccb2142aa3b543375d5d 100644
--- a/chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h
+++ b/chrome/browser/page_load_metrics/observers/no_state_prefetch_page_load_metrics_observer.h
@@ -17,12 +17,26 @@ namespace prerender {
 class PrerenderManager;
 }
 
-// Observer recording metrics related to NoStatePrefetch.
+// Records First Contentful Paint metrics related to NoStatePrefetch.
+//
+// This observer should be attached to all WebContents instances that are not
+// being prerendered. This is because the metrics it records include the boolean
+// bit to tell whether the page was recently eligible for prefetching with
+// NoStatePrefetch prior to the FCP. A prefetched (with NoStatePrefetch) page
+// would load just like a normal new WebContents without knowledge of previous
+// prefetches. The fact that it was prefetched can only be discovered by asking
+// the PrerenderManager at the FCP time.
+//
+// Attaching to all non-prerendered WebContents allows to collect these metrics
+// in other experimental groups (for Prerender and for the simple load) to
+// record how these groups perform on web pages that would otherwise have been
+// prefetched with NostatePrefetch.
 class NoStatePrefetchPageLoadMetricsObserver
     : public page_load_metrics::PageLoadMetricsObserver {
  public:
   // Returns a NoStatePrefetchPageLoadMetricsObserver, or nullptr if it is not
-  // needed.
+  // needed. Note: can return nullptr at startup, which is believed to be
+  // happening rarely.
   static std::unique_ptr<NoStatePrefetchPageLoadMetricsObserver> CreateIfNeeded(
       content::WebContents* web_contents);