[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) {
+  if (!started_in_foreground)

[Charlie Harrison, 2016/11/21 14:30:17]

I would have thought all prerenders start in the background. Don't we want
was_hidden_ to be true only after we've been swapped in and then backgrounded?

[mattcary, 2016/11/21 16:13:37]

That's a good point. Done.

+    was_hidden_ = true;
+
+  start_ticks_ = navigation_handle->NavigationStart();
+  return CONTINUE_OBSERVING;
+}
+
+void PrerenderPageLoadMetricsObserver::OnFirstContentfulPaint(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& extra_info) {
+  if (start_ticks_.is_null()) {

[Charlie Harrison, 2016/11/21 14:30:17]

When will this happen?

[mattcary, 2016/11/21 16:13:37]

If OnStart() doesn't get called. When could that happen? Probably never, now,
but maybe in the future? At any rate, I don't want to pollute our histograms
with strange data, and it seems extreme to check-fail.

[Bryan McQuade, 2016/11/21 16:26:47]

We'll always call OnStart once, for all observers. I'd suggest at least
DCHECKing that this can't happen. Perhaps also log an UMA error counter if you
want to know how often this happens (should be zero times).

[mattcary, 2016/11/23 13:15:17]

DCHECK'd

+    prerender_manager_->RecordPerceivedFirstContentfulPaint(
+        web_contents_, base::TimeTicks(), was_hidden_);
+  } else {
+    prerender_manager_->RecordPerceivedFirstContentfulPaint(
+        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;
+}