Make sure that when a SupportsUserData is destroyed, objects destroyed transitively see it as empty.

base/supports_user_data.cc

 // Copyright (c) 2012 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.
 
 #include "base/supports_user_data.h"
 
 namespace base {
 
 SupportsUserData::SupportsUserData() {
   // Harmless to construct on a different thread to subsequent usage.
@@ skipping 17 matching lines @@
   DCHECK(thread_checker_.CalledOnValidThread());
   user_data_.erase(key);
 }
 
 void SupportsUserData::DetachUserDataThread() {
   thread_checker_.DetachFromThread();
 }
 
 SupportsUserData::~SupportsUserData() {
   DCHECK(thread_checker_.CalledOnValidThread() || user_data_.empty());
+  DataMap local_user_data;
+  user_data_.swap(local_user_data);
+  // Now this->user_data_ is empty, and any destructors called transitively from
+  // the destruction of |local_user_data| will see it that way instead of
+  // examining a being-destroyed object.
 }
 
 }  // namespace base