[Background Fetch] Cleanup/fix thread safety

content/browser/background_fetch/background_fetch_job_controller_unittest.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_job_controller.h"
 
 #include <memory>
 #include <string>
 #include <unordered_map>
 
@@ skipping 66 matching lines @@
     for (const auto& pair : request_data) {
       CreateRequestWithProvidedResponse(
           pair.second, pair.first,
           TestResponseBuilder(200 /* response_code */).Build());
     }
   }
 
   // Creates a new BackgroundFetchJobController instance.
   std::unique_ptr<BackgroundFetchJobController> CreateJobController(
       const BackgroundFetchRegistrationId& registration_id) {
+    // StoragePartition creates its own BackgroundFetchContext, but this test
+    // doesn't use that; it just uses the StoragePartition's URLRequestContext.
     StoragePartition* storage_partition =
         BrowserContext::GetDefaultStoragePartition(browser_context());
 
     return base::MakeUnique<BackgroundFetchJobController>(
         registration_id, BackgroundFetchOptions(), &data_manager_,
         browser_context(),
         make_scoped_refptr(storage_partition->GetURLRequestContext()),
         base::BindOnce(&BackgroundFetchJobControllerTest::DidCompleteJob,
                        base::Unretained(this)),
         TRAFFIC_ANNOTATION_FOR_TESTS);
@@ skipping 139 matching lines @@
 
   // TODO(peter): Verify that the issued download items have had their state
   // updated to be cancelled as well.
 
   EXPECT_EQ(controller->state(), BackgroundFetchJobController::State::ABORTED);
   EXPECT_TRUE(did_complete_job_);
 }
 
 }  // namespace
 }  // namespace content