[Background Fetch] Cleanup/fix thread safety

content/browser/background_fetch/background_fetch_data_manager.h

Index: content/browser/background_fetch/background_fetch_data_manager.h
diff --git a/content/browser/background_fetch/background_fetch_data_manager.h b/content/browser/background_fetch/background_fetch_data_manager.h
index 60b39ffc1fb7f3193972a446933106bfc480bf37..ab17f484b294035ae86e61cab08e76cd929cb171 100644
--- a/content/browser/background_fetch/background_fetch_data_manager.h
+++ b/content/browser/background_fetch/background_fetch_data_manager.h
@@ -13,6 +13,7 @@
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/optional.h"
+#include "base/sequence_checker.h"
 #include "content/browser/background_fetch/background_fetch_registration_id.h"
 #include "content/common/content_export.h"
 #include "third_party/WebKit/public/platform/modules/background_fetch/background_fetch.mojom.h"
@@ -96,6 +97,8 @@ class CONTENT_EXPORT BackgroundFetchDataManager {
   std::map<BackgroundFetchRegistrationId, std::unique_ptr<RegistrationData>>
       registrations_;
 
+  SEQUENCE_CHECKER(sequence_checker_);
+
   base::WeakPtrFactory<BackgroundFetchDataManager> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(BackgroundFetchDataManager);