[Prerender] first contentful paint histograms.

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.
+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>

[pasko, 2016/11/21 14:44:36]

nit: Add #include <memory> for unique_ptr<>  [build/include_what_you_use] [4]

+  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,
+                                   content::WebContents* web_contents);
+
+  // page_load_metrics::PageLoadMetricsObserver:
+  ObservePolicy OnStart(content::NavigationHandle* navigation_handle,
+                        const GURL& currently_committed_url,
+                        bool started_in_foreground) override;
+
+  // page_load_metrics::PageLoadMetricsObserver:
+  void OnFirstContentfulPaint(
+      const page_load_metrics::PageLoadTiming& timing,
+      const page_load_metrics::PageLoadExtraInfo& extra_info) override;
+
+  // page_load_metrics::PageLoadMetricsObserver:
+  ObservePolicy OnHidden(
+      const page_load_metrics::PageLoadTiming& timing,
+      const page_load_metrics::PageLoadExtraInfo& extra_info) override;
+
+ protected:
+  void SetNavigationStartTicksForTesting(base::TimeTicks ticks);
+
+ private:
+  prerender::PrerenderManager* const prerender_manager_;

[pasko, 2016/11/21 14:44:36]

that const is nice, I did not know that the style encourages emphasizing
constness of members after construction. Thanks.

+  content::WebContents* web_contents_;
+  base::TimeTicks start_ticks_;
+  bool was_hidden_;
+
+  DISALLOW_COPY_AND_ASSIGN(PrerenderPageLoadMetricsObserver);
+};
+
+#endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_PRERENDER_PAGE_LOAD_METRICS_OBSERVER_H_