[Background Fetch] Cleanup/fix thread safety

content/browser/background_fetch/background_fetch_event_dispatcher.cc

 // Copyright 2017 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 "content/browser/background_fetch/background_fetch_event_dispatcher.h"
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/metrics/histogram_functions.h"
 #include "base/strings/stringprintf.h"
@@ skipping 47 matching lines @@
 
   // Used because the |histogram_name| is not a constant.
   base::UmaHistogramEnumeration(histogram_name, service_worker_status,
                                 SERVICE_WORKER_ERROR_MAX_VALUE);
 }
 
 }  // namespace
 
 BackgroundFetchEventDispatcher::BackgroundFetchEventDispatcher(
     const scoped_refptr<ServiceWorkerContextWrapper>& service_worker_context)
-    : service_worker_context_(service_worker_context) {}
+    : service_worker_context_(service_worker_context) {
+  // Constructed on the UI thread, then lives on the IO thread.
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+}
 
-BackgroundFetchEventDispatcher::~BackgroundFetchEventDispatcher() = default;
+BackgroundFetchEventDispatcher::~BackgroundFetchEventDispatcher() {
+  DCHECK_CURRENTLY_ON(BrowserThread::IO);
+}
 
 void BackgroundFetchEventDispatcher::DispatchBackgroundFetchAbortEvent(
     const BackgroundFetchRegistrationId& registration_id,
     base::Closure finished_closure) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   LoadServiceWorkerRegistrationForDispatch(
       registration_id, ServiceWorkerMetrics::EventType::BACKGROUND_FETCH_ABORT,
       std::move(finished_closure),
       base::Bind(
           &BackgroundFetchEventDispatcher::DoDispatchBackgroundFetchAbortEvent,
@@ skipping 153 matching lines @@
       } else {
         RecordDispatchResult(event, DISPATCH_RESULT_SUCCESS);
       }
       break;
   }
 
   finished_closure.Run();
 }
 
 }  // namespace content