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

third_party/WebKit/Source/platform/heap/GarbageCollected.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef GarbageCollected_h
 #define GarbageCollected_h
 
 #include "platform/heap/ThreadState.h"
 #include "wtf/Allocator.h"
 #include "wtf/Assertions.h"
@@ skipping 179 matching lines @@
 //
 // The "operator new" for B will enter the forbidden GC scope, but
 // upon construction, two GarbageCollectedMixinConstructorMarker constructors
 // will run -- one for A (first) and another for B (secondly). Only
 // the second one should leave the forbidden GC scope. This is realized by
 // recording the address of B's GarbageCollectedMixinConstructorMarker
 // when the "operator new" for B runs, and leaving the forbidden GC scope
 // when the constructor of the recorded GarbageCollectedMixinConstructorMarker
 // runs.
 #define USING_GARBAGE_COLLECTED_MIXIN(TYPE)                             \
+    IS_GARBAGE_COLLECTED_TYPE();                                        \
     DEFINE_GARBAGE_COLLECTED_MIXIN_METHODS(blink::Visitor*, TYPE)       \
     DEFINE_GARBAGE_COLLECTED_MIXIN_METHODS(blink::InlinedGlobalMarkingVisitor, TYPE) \
     DEFINE_GARBAGE_COLLECTED_MIXIN_CONSTRUCTOR_MARKER(TYPE)             \
 public:                                                                 \
     bool isHeapObjectAlive() const override                             \
     {                                                                   \
         return Heap::isHeapObjectAlive(this);                           \
     }                                                                   \
 private:
 
@@ skipping 185 matching lines @@
 
 namespace WTF {
 
 // Adoption is not needed nor wanted for RefCountedGarbageCollected<>-derived types.
 template<typename T>
 PassRefPtr<T> adoptRef(blink::RefCountedGarbageCollected<T>*) = delete;
 
 } // namespace WTF
 
 #endif