Mojo Bindings: Fix lock-order inversion in associated controllers

mojo/public/cpp/bindings/lib/multiplex_router.h

Index: mojo/public/cpp/bindings/lib/multiplex_router.h
diff --git a/mojo/public/cpp/bindings/lib/multiplex_router.h b/mojo/public/cpp/bindings/lib/multiplex_router.h
index 6ecae85b6ef831d7fe876dfa2737d12d5445eeb3..8a6e2ed28b0374b8678eb64bf94fff8725b6f22c 100644
--- a/mojo/public/cpp/bindings/lib/multiplex_router.h
+++ b/mojo/public/cpp/bindings/lib/multiplex_router.h
@@ -242,6 +242,8 @@ class MOJO_CPP_BINDINGS_EXPORT MultiplexRouter
   // Sets to nullptr in Config::SINGLE_INTERFACE* mode.
   std::unique_ptr<base::Lock> lock_;
   PipeControlMessageHandler control_message_handler_;
+
+  // NOTE: It is unsafe to call into this object while holding |lock_|.
   PipeControlMessageProxy control_message_proxy_;
 
   std::map<InterfaceId, scoped_refptr<InterfaceEndpoint>> endpoints_;