Add missing comments about keeping WeakPtrFactory last.

Move it to last position where it was not.

Change-Id: Ib89f45b250fdaae1056e9b1dbec18f785c72d4b5
diff --git a/src/traced/probes/ftrace/ftrace_controller.h b/src/traced/probes/ftrace/ftrace_controller.h
index 5201e34..275b888 100644
--- a/src/traced/probes/ftrace/ftrace_controller.h
+++ b/src/traced/probes/ftrace/ftrace_controller.h
@@ -129,8 +129,8 @@
   std::vector<std::unique_ptr<CpuReader>> cpu_readers_;
   std::set<FtraceDataSource*> data_sources_;
   std::set<FtraceDataSource*> started_data_sources_;
-  base::WeakPtrFactory<FtraceController> weak_factory_;  // Keep last.
   PERFETTO_THREAD_CHECKER(thread_checker_)
+  base::WeakPtrFactory<FtraceController> weak_factory_;  // Keep last.
 };
 
 }  // namespace perfetto