Skip nullptr after copyToVector() from HeapHashSet<WeakMember>

third_party/WebKit/Source/platform/fonts/FontCache.cpp

Index: third_party/WebKit/Source/platform/fonts/FontCache.cpp
diff --git a/third_party/WebKit/Source/platform/fonts/FontCache.cpp b/third_party/WebKit/Source/platform/fonts/FontCache.cpp
index 9208ac4499d472e7050295c73f450646bdf44527..5b9e9c2edc47e8e4afb9b7e7f274a1067c3c0d80 100644
--- a/third_party/WebKit/Source/platform/fonts/FontCache.cpp
+++ b/third_party/WebKit/Source/platform/fonts/FontCache.cpp
@@ -451,8 +451,12 @@ void FontCache::invalidate() {
 
   HeapVector<Member<FontCacheClient>> clients;
   copyToVector(fontCacheClients(), clients);
-  for (const auto& client : clients)
-    client->fontCacheInvalidated();
+  for (const auto& client : clients) {
+    // This should not be nullptr, but to see if checking nullptr can suppress
+    // crashes. crbug.com/581698
+    if (client)
+      client->fontCacheInvalidated();
+  }
 
   purge(ForcePurge);
 }