Make WTF::IsGarbageCollectedType<> work for GC mixin instances.

third_party/WebKit/Source/platform/heap/HeapTest.cpp

 /*
  * 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 1520 matching lines @@
     DEFINE_INLINE_VIRTUAL_TRACE()
     {
         SimpleObject::trace(visitor);
         Mixin::trace(visitor);
         ++s_traceCount;
     }
 
 private:
     UseMixin()
     {
+        // Verify that WTF::IsGarbageCollectedType<> works as expected for mixins.
+        static_assert(WTF::IsGarbageCollectedType<UseMixin>::value, "IsGarbageCollectedType<> sanity check failed for GC mixin.");
         s_traceCount = 0;
     }
 };
 
 int UseMixin::s_traceCount = 0;
 
 class VectorObject {
     DISALLOW_NEW_EXCEPT_PLACEMENT_NEW();
 public:
     VectorObject()
@@ skipping 4950 matching lines @@
     EXPECT_EQ(1u, vector2.size());
 // TODO(Oilpan): when Vector.h's contiguous container support no longer disables
 // Vector<>s with inline capacity, remove.
 #if !defined(ANNOTATE_CONTIGUOUS_CONTAINER)
     EXPECT_EQ(16u, vector1.capacity());
     EXPECT_EQ(16u, vector2.capacity());
 #endif
 }
 
 } // namespace blink