[Background Fetch] Cleanup/fix thread safety

content/browser/background_fetch/background_fetch_test_base.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_test_base.h"
 
 #include <stdint.h>
 #include <memory>
 #include <utility>
 
@@ skipping 183 matching lines @@
 
   // Temporary directory in which successfully downloaded files will be stored.
   base::ScopedTempDir temp_directory_;
 
   base::WeakPtrFactory<RespondingDownloadManager> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(RespondingDownloadManager);
 };
 
 BackgroundFetchTestBase::BackgroundFetchTestBase()
+    // Using REAL_IO_THREAD would give better coverage for thread safety, but
+    // at time of writing EmbeddedWorkerTestHelper didn't seem to support that.
     : thread_bundle_(TestBrowserThreadBundle::IO_MAINLOOP),
       origin_(GURL(kTestOrigin)) {}
 
 BackgroundFetchTestBase::~BackgroundFetchTestBase() {
   DCHECK(set_up_called_);
   DCHECK(tear_down_called_);
 }
 
 void BackgroundFetchTestBase::SetUp() {
   download_manager_ = new RespondingDownloadManager();
@@ skipping 79 matching lines @@
   // Create a ServiceWorkerFetchRequest request with the same information.
   return ServiceWorkerFetchRequest(gurl, method, ServiceWorkerHeaderMap(),
                                    Referrer(), false /* is_reload */);
 }
 
 MockDownloadManager* BackgroundFetchTestBase::download_manager() {
   return download_manager_;
 }
 
 }  // namespace content