Skip nullptr after copyToVector() from HeapHashSet<WeakMember>

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

Index: third_party/WebKit/Source/core/css/CSSFontSelector.cpp
diff --git a/third_party/WebKit/Source/core/css/CSSFontSelector.cpp b/third_party/WebKit/Source/core/css/CSSFontSelector.cpp
index a5970731553f047a430e143a04a386c9331c6220..48bc761b5477f9c65b50c18b90db82a814de017e 100644
--- a/third_party/WebKit/Source/core/css/CSSFontSelector.cpp
+++ b/third_party/WebKit/Source/core/css/CSSFontSelector.cpp
@@ -73,8 +73,12 @@ void CSSFontSelector::dispatchInvalidationCallbacks() {
 
   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() {