Followup comments from r460581.

third_party/WebKit/Source/core/frame/LocalDOMWindow.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 1407 matching lines @@
 
   for (auto& it : m_eventListenerObservers) {
     it->didAddEventListener(this, eventType);
   }
 
   if (eventType == EventTypeNames::unload) {
     UseCounter::count(document(), UseCounter::DocumentUnloadRegistered);
     addUnloadEventListener(this);
   } else if (eventType == EventTypeNames::beforeunload) {
     UseCounter::count(document(), UseCounter::DocumentBeforeUnloadRegistered);
+    // This is confusingly named. It doesn't actually add the listener. It
+    // just increments a count so that we know we have listeners registered
+    // for the purposes of determining if we can fast terminate the renderer
+    // process.

[dcheng, 2017/03/30 19:16:57]

Nit: move this to line 1423 (since it applies equally to addUnloadEventListener
as well)

     addBeforeUnloadEventListener(this);
     if (frame() && !frame()->isMainFrame())
       UseCounter::count(document(), UseCounter::SubFrameBeforeUnloadRegistered);
   }
 }
 
 void LocalDOMWindow::removedEventListener(
     const AtomicString& eventType,
     const RegisteredEventListener& registeredListener) {
   DOMWindow::removedEventListener(eventType, registeredListener);
@@ skipping 205 matching lines @@
   DOMWindow::trace(visitor);
   Supplementable<LocalDOMWindow>::trace(visitor);
 }
 
 DEFINE_TRACE_WRAPPERS(LocalDOMWindow) {
   visitor->traceWrappers(m_customElements);
   DOMWindow::traceWrappers(visitor);
 }
 
 }  // namespace blink