Allow null other.m_parent in TraceWrapperMember::operator= if other is nullptr.

third_party/WebKit/Source/bindings/core/v8/TraceWrapperMember.h

 // Copyright 2016 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 TraceWrapperMember_h
 #define TraceWrapperMember_h
 
 #include "bindings/core/v8/ScriptWrappableVisitor.h"
 #include "platform/heap/HeapAllocator.h"
 
@@ skipping 32 matching lines @@
   TraceWrapperMember(WTF::HashTableDeletedValueType x)
       : Member<T>(x), m_parent(nullptr) {}
 
   /**
    * Copying a TraceWrapperMember means that its backpointer will also be
    * copied.
    */
   TraceWrapperMember(const TraceWrapperMember& other) { *this = other; }
 
   TraceWrapperMember& operator=(const TraceWrapperMember& other) {
-    DCHECK(other.m_parent);
+    DCHECK(!other.m_raw || other.m_parent);
     m_parent = other.m_parent;
     Member<T>::operator=(other);
     ScriptWrappableVisitor::writeBarrier(m_parent, other);
     return *this;
   }
 
   TraceWrapperMember& operator=(const Member<T>& other) {
     DCHECK(!traceWrapperMemberIsNotInitialized());
     Member<T>::operator=(other);
     ScriptWrappableVisitor::writeBarrier(m_parent, other);
@@ skipping 74 matching lines @@
   b.clear();
   b.reserveCapacity(temp.size());
   for (auto item : temp) {
     b.push_back(item.get());
   }
 }
 
 }  // namespace blink
 
 #endif  // TraceWrapperMember_h