Only check for on-heap Member<T>s iff T is in scope.

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/Assertions.h"
 #include "wtf/ListHashSet.h"
@@ skipping 404 matching lines @@
 template <typename T> const bool NeedsAdjustAndMark<T, true>::value;
 
 template<typename T>
 class NeedsAdjustAndMark<T, false> {
     static_assert(sizeof(T), "T must be fully defined");
 public:
     static const bool value = IsGarbageCollectedMixin<typename WTF::RemoveConst<T>::Type>::value;
 };
 template <typename T> const bool NeedsAdjustAndMark<T, false>::value;
 
+// TODO(sof): migrate to wtf/TypeTraits.h
+template<typename T>
+class IsFullyDefined {
+    using TrueType = char;
+    struct FalseType {
+        char dummy[2];
+    };
+
+    template<typename U, size_t sz = sizeof(U)> static TrueType isSizeofKnown(U*);
+    static FalseType isSizeofKnown(...);
+    static T& t;
+public:
+    static const bool value = sizeof(TrueType) == sizeof(isSizeofKnown(&t));
+};
+
 } // namespace blink
 
 #endif