[S] Use a weak_factory for V4Database callbacks + test

components/safe_browsing_db/v4_database.h

Index: components/safe_browsing_db/v4_database.h
diff --git a/components/safe_browsing_db/v4_database.h b/components/safe_browsing_db/v4_database.h
index 19fee93a53b304007a69fef142473b93c5826ba2..cc07d4e308a8be28944a2ba2e84e0d7eaae50fde 100644
--- a/components/safe_browsing_db/v4_database.h
+++ b/components/safe_browsing_db/v4_database.h
@@ -8,6 +8,7 @@
 #include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
 #include "base/sequenced_task_runner.h"
 #include "base/single_thread_task_runner.h"
 #include "components/safe_browsing_db/v4_protocol_manager_util.h"
@@ -205,6 +206,10 @@ class V4Database {
   // accessed on the IO thread.
   int pending_store_updates_;
 
+  // Only meant to be dereferenced and invalidated on the IO thread and hence
+  // named. For details, see the comment at the top of weak_ptr.h
+  base::WeakPtrFactory<V4Database> weak_factory_on_io_;
+
   DISALLOW_COPY_AND_ASSIGN(V4Database);
 };