Instantiate PageLoadTrackers for prerenders

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 b3454624fef18d7fea059784ccc04c1bc812c671..f42b5746261ee1414d68dafa02b13bb5fcbbe57f 100644
--- a/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
+++ b/chrome/browser/page_load_metrics/metrics_web_contents_observer.cc
@@ -301,6 +301,15 @@ PageLoadTracker::PageLoadTracker(
       aborted_chain_size_same_url_(aborted_chain_size_same_url),
       embedder_interface_(embedder_interface) {
   DCHECK(!navigation_handle->HasCommitted());
+  if (embedder_interface_->IsPrerendering(
+          navigation_handle->GetWebContents())) {
+    DCHECK(!started_in_foreground_);
+    // For the time being, we do not track prerenders. See crbug.com/648338 for
+    // details.
+    StopTracking();
+    return;
+  }
+
   embedder_interface_->RegisterObservers(this);
   for (const auto& observer : observers_) {
     observer->OnStart(navigation_handle, currently_committed_url,
@@ -756,11 +765,6 @@ void MetricsWebContentsObserver::WillStartNavigationRequest(
   if (!ShouldTrackNavigation(navigation_handle))
     return;
 
-  // TODO(bmcquade): add support for tracking prerendered pages when they become
-  // visible.
-  if (embedder_interface_->IsPrerendering(web_contents()))
-    return;
-
   // Pass in the last committed url to the PageLoadTracker. If the MWCO has
   // never observed a committed load, use the last committed url from this
   // WebContent's opener. This is more accurate than using referrers due to