Mark some content::BrowserThread methods with WARN_UNUSED_RESULT.

content/browser/service_worker/service_worker_test_utils.h

 // 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.
 
 #ifndef CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_TEST_UTILS_H_
 #define CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_TEST_UTILS_H_
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "content/public/browser/browser_thread.h"
@@ skipping 11 matching lines @@
 
 template <typename Arg> base::Callback<void(Arg)>
 CreateReceiver(BrowserThread::ID run_quit_thread,
                const base::Closure& quit, Arg* out) {
   return base::Bind(&ReceiveResult<Arg>, run_quit_thread, quit, out);
 }
 
 template <typename Arg> base::Callback<void(Arg)>
 CreateReceiverOnCurrentThread(Arg* out) {
   BrowserThread::ID id;
-  BrowserThread::GetCurrentThreadIdentifier(&id);
+  bool ret = BrowserThread::GetCurrentThreadIdentifier(&id);
+  DCHECK(ret);
   return base::Bind(&ReceiveResult<Arg>, id, base::Closure(), out);
 }
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_SERVICE_WORKER_SERVICE_WORKER_TEST_UTILS_H_