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

Source/platform/heap/GarbageCollected.h

Index: Source/platform/heap/GarbageCollected.h
diff --git a/Source/platform/heap/GarbageCollected.h b/Source/platform/heap/GarbageCollected.h
index ce920621a9261b50b4a0f51985a43131018340b0..5fe5388fcf849b7c7aebaa4f4aaa1e4800a0b04c 100644
--- a/Source/platform/heap/GarbageCollected.h
+++ b/Source/platform/heap/GarbageCollected.h
@@ -422,6 +422,21 @@ public:
 };
 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