Check not to insert nullptr to 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..2ab381d572e9287d5ff447e56f1186c9c781e2bf 100644
--- a/third_party/WebKit/Source/platform/fonts/FontCache.cpp
+++ b/third_party/WebKit/Source/platform/fonts/FontCache.cpp
@@ -426,6 +426,7 @@ HeapHashSet<WeakMember<FontCacheClient>>& fontCacheClients() {
 }
 
 void FontCache::addClient(FontCacheClient* client) {
+  CHECK(client);
   ASSERT(!fontCacheClients().contains(client));
   fontCacheClients().insert(client);
 }