[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>
+  CreateIfNeeded(content::WebContents* web_contents);
+
+  explicit PrerenderPageLoadMetricsObserver(

[Bryan McQuade, 2016/11/18 14:52:37]

given the static createifneeded method. does this need to also be public? if
public only for tests, let's add a comment to note that.

[mattcary, 2016/11/18 15:14:14]

Done.

+      prerender::PrerenderManager* manager,

[Bryan McQuade, 2016/11/18 14:52:37]

just want to make sure PrerenderManager's lifetime is guaranteed to exceed the
lifetime of the observer - is that right? what's the lifetime of
PrerenderManager? Maybe add a short comment to note it.

[mattcary, 2016/11/18 15:14:14]

Done.

The PrerenderManager lifetime is like that of the browser process; it has to
outlive all the renderers and web contents as it manages who gets swapped in
where.

+      content::WebContents* web_contents);
+
+  // page_load_metrics::PageLoadMetricsObserver:
+  void OnFirstContentfulPaint(
+      const page_load_metrics::PageLoadTiming& timing,
+      const page_load_metrics::PageLoadExtraInfo& extra_info) override;
+
+ private:
+  prerender::PrerenderManager* const prerender_manager_;
+  content::WebContents* web_contents_;
+
+  DISALLOW_COPY_AND_ASSIGN(PrerenderPageLoadMetricsObserver);
+};
+
+#endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_PRERENDER_PAGE_LOAD_METRICS_OBSERVER_H_