Prerender: clarify how FCP works in non-prerendered cases

chrome/browser/page_load_metrics/observers/prerender_page_load_metrics_observer.h

 // Copyright 2016 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_PAGE_LOAD_METRICS_OBSERVERS_PRERENDER_PAGE_LOAD_METRICS_OBSERVER_H_
 #define CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_PRERENDER_PAGE_LOAD_METRICS_OBSERVER_H_
 
 #include "base/macros.h"
 #include "chrome/browser/page_load_metrics/page_load_metrics_observer.h"
 #include "url/gurl.h"
 
 namespace content {
 class WebContents;
 }
 
 namespace prerender {
 class PrerenderManager;
 }
 
-// Observer recording metrics related to prerender.
+// Observer responsible for recording First Contentful Paing metrics related to
+// Prerender.
+//
+// To record FCP metrics for non-Prerender loads, the
+// |NoStatePrefetchPageLoadMetricsObserver| is used.
 class PrerenderPageLoadMetricsObserver
     : public page_load_metrics::PageLoadMetricsObserver {
  public:
   // Returns a PrerenderPageLoadMetricsObserver, or nullptr if it is not needed.
   static std::unique_ptr<page_load_metrics::PageLoadMetricsObserver>
   CreateIfNeeded(content::WebContents* web_contents);
 
   // Public for testing. Normally one should use CreateIfNeeded. Both the
   // manager and the web_contents must outlive this observer.
   PrerenderPageLoadMetricsObserver(prerender::PrerenderManager* manager,
@@ skipping 17 matching lines @@
   prerender::PrerenderManager* const prerender_manager_;
   content::WebContents* web_contents_;
   base::TimeTicks start_ticks_;
   bool is_no_store_;
   bool was_hidden_;
 
   DISALLOW_COPY_AND_ASSIGN(PrerenderPageLoadMetricsObserver);
 };
 
 #endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_PRERENDER_PAGE_LOAD_METRICS_OBSERVER_H_