heap: Don't call write barriers from UntracedMembers assign operators

Templated assignment operators don't hide the assignment operator which
is a special function. In the case of UntracedMember the assignment
operator was generated by the compiler, which was

Bug: 1021889
Change-Id: I835a444753449096b01b6ee2087149360543d3cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2001304Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731894}

third_party/blink/renderer/platform/heap/member.h

@@ -398,6 +398,12 @@ class UntracedMember final
 
   UntracedMember(WTF::HashTableDeletedValueType x) : Parent(x) {}
 
+  UntracedMember& operator=(const UntracedMember& other) {
+    this->SetRaw(other);
+    this->CheckPointer();
+    return *this;
+  }
+
   template <typename U>
   UntracedMember& operator=(const Persistent<U>& other) {
     this->SetRaw(other);