Prevent cross-thread refcounting for thread-unsafe IndexedDBCallbacks.

content/browser/indexed_db/indexed_db_database_callbacks.h

Index: content/browser/indexed_db/indexed_db_database_callbacks.h
diff --git a/content/browser/indexed_db/indexed_db_database_callbacks.h b/content/browser/indexed_db/indexed_db_database_callbacks.h
index 5ea8cd7882e1ef7699c1484a91c4cc9e15b22c6b..e95a62324126c651f324b0a76f843afaef402480 100644
--- a/content/browser/indexed_db/indexed_db_database_callbacks.h
+++ b/content/browser/indexed_db/indexed_db_database_callbacks.h
@@ -9,6 +9,7 @@
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/threading/thread_checker.h"
 #include "content/common/content_export.h"
 #include "content/common/indexed_db/indexed_db.mojom.h"
 #include "content/public/browser/browser_thread.h"
@@ -47,6 +48,7 @@ class CONTENT_EXPORT IndexedDBDatabaseCallbacks
   scoped_refptr<IndexedDBDispatcherHost> dispatcher_host_;
   int32_t ipc_thread_id_;
   std::unique_ptr<IOThreadHelper, BrowserThread::DeleteOnIOThread> io_helper_;
+  base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(IndexedDBDatabaseCallbacks);
 };