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..0fdd2573edf17ba32d76abd8380c8da33c1b533f 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 +17,24 @@ namespace prerender {
 class PrerenderManager;
 }
 
-// Observer recording metrics related to NoStatePrefetch.
+// Observer responsible for recording First Contentful Paint metrics related to
+// NoStatePrefetch.
+//
+// This observer should be attached to all WebContents instances that are _not_
+// being prerendered. For prerendered page loads, analagous metrics are recorded
+// via |PrerenderPageLoadMetricsObserver|. This allows to compare FCP metrics
+// between three mechanisms: Prerender, NoStatePrefetch, Noop.
+//
+// To record the histograms the knowledge of this class is combined with
+// information from |PrerenderManager|:
+//   * the kind of prefetch, i.e.: prerender::Origin
+//   * whether the load was eligible for prefetch/prerender (also how long ago)
 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);