Remove unused thread.h include in trace_event_impl.h and fix IWUU.

components/policy/core/common/cloud/policy_header_service_unittest.cc

 // Copyright 2013 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 "components/policy/core/common/cloud/policy_header_service.h"
 
 #include <memory>
 #include <utility>
 
 #include "base/base64.h"
 #include "base/json/json_reader.h"
+#include "base/message_loop/message_loop.h"
 #include "base/test/test_simple_task_runner.h"
 #include "base/values.h"
 #include "components/policy/core/common/cloud/cloud_policy_constants.h"
 #include "components/policy/core/common/cloud/mock_cloud_policy_store.h"
 #include "components/policy/core/common/cloud/policy_header_io_helper.h"
 #include "net/http/http_request_headers.h"
 #include "net/url_request/url_request.h"
 #include "net/url_request/url_request_test_util.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 99 matching lines @@
   user_store_.SetPolicy(std::unique_ptr<PolicyData>());
   task_runner_->RunUntilIdle();
 
   std::unique_ptr<net::URLRequest> request2(
       context.CreateRequest(GURL(kDMServerURL), net::DEFAULT_PRIORITY, NULL));
   helper_->AddPolicyHeaders(request2->url(), request2.get());
   ValidateHeader(request2->extra_request_headers(), "", "");
 }
 
 }  // namespace policy