Skip nullptr after copyToVector() from HeapHashSet<WeakMember>

third_party/WebKit/Source/core/css/CSSFontSelector.cpp

 /*
  * Copyright (C) 2007, 2008, 2011 Apple Inc. All rights reserved.
  *           (C) 2007, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  *
  * 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 55 matching lines @@
 void CSSFontSelector::unregisterForInvalidationCallbacks(
     CSSFontSelectorClient* client) {
   m_clients.erase(client);
 }
 
 void CSSFontSelector::dispatchInvalidationCallbacks() {
   m_fontFaceCache.incrementVersion();
 
   HeapVector<Member<CSSFontSelectorClient>> clients;
   copyToVector(m_clients, clients);
-  for (auto& client : clients)
-    client->fontsNeedUpdate(this);
+  for (auto& client : clients) {
+    // This should not be nullptr, but to see if checking nullptr can suppress
+    // crashes. crbug.com/581698
+    if (client)
+      client->fontsNeedUpdate(this);
+  }
 }
 
 void CSSFontSelector::fontFaceInvalidated() {
   dispatchInvalidationCallbacks();
 }
 
 void CSSFontSelector::fontCacheInvalidated() {
   dispatchInvalidationCallbacks();
 }
 
@@ skipping 85 matching lines @@
 }
 
 DEFINE_TRACE(CSSFontSelector) {
   visitor->trace(m_document);
   visitor->trace(m_fontFaceCache);
   visitor->trace(m_clients);
   FontSelector::trace(visitor);
 }
 
 }  // namespace blink