Buffer cross frame paint timing updates.

chrome/browser/page_load_metrics/metrics_web_contents_observer.cc

Index: chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
diff --git a/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc b/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
index 647cf08d1c529b8f09895419a9fef53b6def2b53..2398fc4c5efba79fd64480c599037cf5b8ffea17 100644
--- a/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
+++ b/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
@@ -93,14 +93,23 @@ MetricsWebContentsObserver* MetricsWebContentsObserver::CreateForWebContents(
   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)