[oilpan] Null pointers must be treated as live, since they can't be marked live by strongification

Source/platform/heap/Visitor.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 367 matching lines @@
 #if ENABLE(ASSERT)
     virtual bool weakTableRegistered(const void*) = 0;
 #endif
 
     virtual bool isMarked(const void*) = 0;
 
     template<typename T> inline bool isAlive(T* obj)
     {
         // Check that we actually know the definition of T when tracing.
         COMPILE_ASSERT(sizeof(T), WeNeedToKnowTheDefinitionOfTheTypeWeAreTracing);
-        return !!obj && ObjectAliveTrait<T>::isAlive(this, obj);
+        // The strongification of collections relies on the fact that once a
+        // collection has been strongified, there is no way that it can contain
+        // non-live entries, so no entries will be removed. Since you can't set
+        // the mark bit on a null pointer, that means that null pointers are
+        // always 'alive'.
+        if (!obj)
+            return true;
+        return ObjectAliveTrait<T>::isAlive(this, obj);
     }
     template<typename T> inline bool isAlive(const Member<T>& member)
     {
         return isAlive(member.get());
     }
     template<typename T> inline bool isAlive(RawPtr<T> ptr)
     {
         return isAlive(ptr.get());
     }
 
@@ skipping 235 matching lines @@
 struct GCInfoTrait {
     static const GCInfo* get()
     {
         return GCInfoAtBase<typename GetGarbageCollectedBase<T>::type>::get();
     }
 };
 
 }
 
 #endif