Buffer cross frame paint timing updates.

chrome/browser/page_load_metrics/metrics_web_contents_observer.cc

 // Copyright 2015 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/metrics_web_contents_observer.h"
 
 #include <algorithm>
 #include <ostream>
 #include <string>
 #include <utility>
@@ skipping 75 matching lines @@
 
   MetricsWebContentsObserver* metrics = FromWebContents(web_contents);
   if (!metrics) {
     metrics = new MetricsWebContentsObserver(web_contents,
                                              std::move(embedder_interface));
     web_contents->SetUserData(UserDataKey(), base::WrapUnique(metrics));
   }
   return metrics;
 }
 
-MetricsWebContentsObserver::~MetricsWebContentsObserver() {
+MetricsWebContentsObserver::~MetricsWebContentsObserver() {}
+
+void MetricsWebContentsObserver::WebContentsDestroyed() {
   // TODO(csharrison): Use a more user-initiated signal for CLOSE.
   NotifyPageEndAllLoads(END_CLOSE, UserInitiatedInfo::NotUserInitiated());
 
+  // We tear down PageLoadTrackers in WebContentsDestroyed, rather than in the
+  // destructor, since |web_contents()| returns nullptr in the destructor, and
+  // PageLoadMetricsObservers can cause code to execute that wants to be able to

[Charlie Harrison, 2017/05/26 19:31:34]

which code? It would be nice to be specific here if possible.

[Bryan McQuade, 2017/05/26 20:07:32]

any access to web_contents() in the context of a WCO will blow up.

the specific case i saw was in some prerender code, but it could happen anywhere
as long as the call stack ends up at code that tries to access the
web_contents() for a WCO, for example.

I'm disinclined to mention the specific case of prerender in this comment,
especially since prerender may be going away in the future, but I'm happy to add
more detail about the general issue if you think that would help.

+  // access the current WebContents.
+  committed_load_ = nullptr;
+  provisional_loads_.clear();
+  aborted_provisional_loads_.clear();
+
   for (auto& observer : testing_observers_)
     observer.OnGoingAway();
 }
-
 void MetricsWebContentsObserver::RegisterInputEventObserver(
     content::RenderViewHost* host) {
   if (host != nullptr)
     host->GetWidget()->AddInputEventObserver(this);
 }
 
 void MetricsWebContentsObserver::UnregisterInputEventObserver(
     content::RenderViewHost* host) {
   if (host != nullptr)
     host->GetWidget()->RemoveInputEventObserver(this);
@@ skipping 560 matching lines @@
     observer_->RemoveTestingObserver(this);
     observer_ = nullptr;
   }
 }
 
 void MetricsWebContentsObserver::TestingObserver::OnGoingAway() {
   observer_ = nullptr;
 }
 
 }  // namespace page_load_metrics