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

base/supports_user_data_unittest.cc

+// Copyright 2014 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"
+
+#include <vector>
+
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+namespace {
+
+struct TestSupportsUserData : public SupportsUserData {};
+
+struct UsesItself : public SupportsUserData::Data {
+  UsesItself(SupportsUserData* supports_user_data, const void* key)
+      : supports_user_data_(supports_user_data),
+        key_(key) {
+  }
+
+  virtual ~UsesItself() {
+    EXPECT_EQ(NULL, supports_user_data_->GetUserData(key_));
+  }
+
+  SupportsUserData* supports_user_data_;
+  const void* key_;
+};
+
+TEST(SupportsUserDataTest, ClearWorksRecursively) {
+  TestSupportsUserData supports_user_data;
+  char key = 0;
+  supports_user_data.SetUserData(&key,
+                                 new UsesItself(&supports_user_data, &key));
+  // Destruction of supports_user_data runs the actual test.
+}
+
+}  // namespace
+}  // namespace base