Oilpan: Ensure that types definitions are known for traced weak members.

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 241 matching lines @@
 
     // WeakMember version of the templated trace method. It doesn't keep
     // the traced thing alive, but will write null to the WeakMember later
     // if the pointed-to object is dead. It's lying for this to be const,
     // but the overloading resolver prioritizes constness too high when
     // picking the correct overload, so all these trace methods have to have
     // the same constness on their argument to allow the type to decide.
     template<typename T>
     void trace(const WeakMember<T>& t)
     {
+        // Check that we actually know the definition of T when tracing.
+        COMPILE_ASSERT(sizeof(T), WeNeedToKnowTheDefinitionOfTheTypeWeAreTracing);
         registerWeakCell(const_cast<WeakMember<T>&>(t).cell());
     }
 
     template<typename T>
     void traceInCollection(T& t, ShouldWeakPointersBeMarkedStrongly strongify)
     {
         HashTraits<T>::traceInCollection(this, t, strongify);
     }
 
     // Fallback trace method for part objects to allow individual trace methods
@@ skipping 475 matching lines @@
 struct GCInfoTrait {
     static const GCInfo* get()
     {
         return GCInfoAtBase<typename GetGarbageCollectedBase<T>::type>::get();
     }
 };
 
 }
 
 #endif