[Prerender] first contentful paint histograms.

chrome/browser/page_load_metrics/observers/prerender_page_load_metrics_observer.cc

+// 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.
+
+#include "chrome/browser/page_load_metrics/observers/prerender_page_load_metrics_observer.h"
+
+#include "base/memory/ptr_util.h"
+#include "chrome/browser/prerender/prerender_manager.h"
+#include "chrome/browser/prerender/prerender_manager_factory.h"
+#include "content/public/browser/web_contents.h"
+
+// static
+std::unique_ptr<page_load_metrics::PageLoadMetricsObserver>
+PrerenderPageLoadMetricsObserver::CreateIfNeeded(
+    content::WebContents* web_contents) {
+  prerender::PrerenderManager* manager =
+      prerender::PrerenderManagerFactory::GetForBrowserContext(
+          web_contents->GetBrowserContext());
+  if (!manager)
+    return nullptr;
+
+  if (manager->PageLoadMetricsObserverDisabledForTesting())
+    return nullptr;
+
+  return base::WrapUnique<page_load_metrics::PageLoadMetricsObserver>(
+      new PrerenderPageLoadMetricsObserver(manager, web_contents));
+}
+
+PrerenderPageLoadMetricsObserver::PrerenderPageLoadMetricsObserver(
+    prerender::PrerenderManager* manager,
+    content::WebContents* web_contents)
+    : prerender_manager_(manager),
+      web_contents_(web_contents),
+      was_hidden_(false) {
+  DCHECK(prerender_manager_);
+}
+
+page_load_metrics::PageLoadMetricsObserver::ObservePolicy
+PrerenderPageLoadMetricsObserver::OnStart(
+    content::NavigationHandle* navigation_handle,
+    const GURL& currently_committed_url,
+    bool started_in_foreground) {
+  DCHECK(!started_in_foreground);
+
+  start_ticks_ = navigation_handle->NavigationStart();
+  return CONTINUE_OBSERVING;
+}
+
+void PrerenderPageLoadMetricsObserver::OnFirstContentfulPaint(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& extra_info) {
+  DCHECK(!start_ticks_.is_null());
+  prerender_manager_->RecordPerceivedFirstContentfulPaint(

[Bryan McQuade, 2016/11/23 13:51:23]

optional: it turns out that, given your implementation, you can get away without
implementing OnHidden, and instead doing the following

bool was_hidden = extra_info.first_background_time &&
    timing.first_contentful_paint.value() <=
extra_info.first_background_time.value();

+      web_contents_, start_ticks_ + *timing.first_contentful_paint,
+      was_hidden_);
+}
+
+page_load_metrics::PageLoadMetricsObserver::ObservePolicy
+PrerenderPageLoadMetricsObserver::OnHidden(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& extra_info) {
+  was_hidden_ = true;
+  return CONTINUE_OBSERVING;
+}
+
+void PrerenderPageLoadMetricsObserver::SetNavigationStartTicksForTesting(
+    base::TimeTicks ticks) {
+  start_ticks_ = ticks;
+}