[Background Fetch] Cleanup/fix thread safety

content/browser/background_fetch/background_fetch_test_base.h

Index: content/browser/background_fetch/background_fetch_test_base.h
diff --git a/content/browser/background_fetch/background_fetch_test_base.h b/content/browser/background_fetch/background_fetch_test_base.h
index 71482cd37c847146eaabb8466aa4d37cacd42916..2bc8f6835c0492219225c24979c7ad358e2aed12 100644
--- a/content/browser/background_fetch/background_fetch_test_base.h
+++ b/content/browser/background_fetch/background_fetch_test_base.h
@@ -100,10 +100,12 @@ class BackgroundFetchTestBase : public ::testing::Test {
   // 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_|