[Background Fetch] Cleanup/fix thread safety

content/browser/background_fetch/background_fetch_test_base.h

 // 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.
 
 #ifndef CONTENT_BROWSER_BACKGROUND_FETCH_BACKGROUND_FETCH_TEST_BASE_H_
 #define CONTENT_BROWSER_BACKGROUND_FETCH_BACKGROUND_FETCH_TEST_BASE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 82 matching lines @@
 
   // Returns the browser context that should be used for the tests.
   BrowserContext* browser_context() { return &browser_context_; }
 
   // Returns the download manager used for the tests.
   MockDownloadManager* download_manager();
 
   // Returns the origin that should be used for Background Fetch tests.
   const url::Origin& origin() const { return origin_; }
 
+ protected:
+  TestBrowserThreadBundle thread_bundle_;  // Must be first member.
+
  private:
   class RespondingDownloadManager;
 
-  TestBrowserThreadBundle thread_bundle_;
   TestBrowserContext browser_context_;
 
   RespondingDownloadManager* download_manager_;  // owned by |browser_context_|
 
   BackgroundFetchEmbeddedWorkerTestHelper embedded_worker_test_helper_;
 
   url::Origin origin_;
 
   // Vector of ServiceWorkerRegistration instances that have to be kept alive
   // for the lifetime of this test.
   std::vector<scoped_refptr<ServiceWorkerRegistration>>
       service_worker_registrations_;
 
   bool set_up_called_ = false;
   bool tear_down_called_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(BackgroundFetchTestBase);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_BACKGROUND_FETCH_BACKGROUND_FETCH_TEST_BASE_H_