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

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

Index: third_party/WebKit/Source/bindings/core/v8/TraceWrapperMember.h
diff --git a/third_party/WebKit/Source/bindings/core/v8/TraceWrapperMember.h b/third_party/WebKit/Source/bindings/core/v8/TraceWrapperMember.h
index 17a499dd2be9c87f9dd47f779add5cadd5d73511..0a55521bc80257b7574e3f4ea1bace04f824c09c 100644
--- a/third_party/WebKit/Source/bindings/core/v8/TraceWrapperMember.h
+++ b/third_party/WebKit/Source/bindings/core/v8/TraceWrapperMember.h
@@ -50,7 +50,7 @@ class TraceWrapperMember : public Member<T> {
   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);