Prevent GCs when constructing GC mixin objects.

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 905 matching lines @@
 #define USING_GARBAGE_COLLECTED_MIXIN(TYPE) \
     DEFINE_GARBAGE_COLLECTED_MIXIN_METHODS(blink::Visitor*, TYPE)
 #endif
 
 #if ENABLE(OILPAN)
 #define WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(TYPE) USING_GARBAGE_COLLECTED_MIXIN(TYPE)
 #else
 #define WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(TYPE)
 #endif
 
+// Template to determine if a class is a GarbageCollectedMixin by checking if it
+// has IsGarbageCollectedMixinMarker
+template<typename T>
+struct IsGarbageCollectedMixin {
+private:
+    typedef char YesType;
+    struct NoType {
+        char padding[8];
+    };
+
+    template <typename U> static YesType checkMarker(typename U::IsGarbageCollectedMixinMarker*);
+    template <typename U> static NoType checkMarker(...);
+
+public:
+    static const bool value = sizeof(checkMarker<T>(nullptr)) == sizeof(YesType);
+};
+
 #if ENABLE(GC_PROFILING)
 template<typename T>
 struct TypenameStringTrait {
     static const String& get()
     {
         DEFINE_STATIC_LOCAL(String, typenameString, (WTF::extractTypeNameFromFunctionName(WTF::extractNameFunction<T>())));
         return typenameString;
     }
 };
 #endif
@@ skipping 63 matching lines @@
 struct GCInfoTrait {
     static size_t index()
     {
         return GCInfoAtBase<typename GetGarbageCollectedBase<T>::type>::index();
     }
 };
 
 }
 
 #endif