Speculatively handle weak member clearing while creating iteration vector.

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 da4476d005dc497f32e01316c19864197b2591fb..1719ae944ed8620057dc18ebeee729455083a8b1 100644
--- a/third_party/WebKit/Source/core/css/CSSFontSelector.cpp
+++ b/third_party/WebKit/Source/core/css/CSSFontSelector.cpp
@@ -83,8 +83,12 @@ void CSSFontSelector::dispatchInvalidationCallbacks()
 
     WillBeHeapVector<RawPtrWillBeMember<CSSFontSelectorClient>> clients;
     copyToVector(m_clients, clients);
-    for (size_t i = 0; i < clients.size(); ++i)
-        clients[i]->fontsNeedUpdate(this);
+    // TODO(sof): the null check is temporarily in place to speculatively address
+    // crbug.com/568173.
+    for (auto& client : clients) {
+        if (client)
+            client->fontsNeedUpdate(this);
+    }
 }
 
 void CSSFontSelector::fontFaceInvalidated()