exo: Recreate Surface resources on context lost.

ash/sysui/stub_context_factory.h

Index: ash/sysui/stub_context_factory.h
diff --git a/ash/sysui/stub_context_factory.h b/ash/sysui/stub_context_factory.h
index 606297d4fc696fc8223a4b536ed61d473a698086..e69741eda613d9111f2ce5e138aba8f19347b25b 100644
--- a/ash/sysui/stub_context_factory.h
+++ b/ash/sysui/stub_context_factory.h
@@ -44,6 +44,8 @@ class StubContextFactory : public ui::ContextFactory {
   void SetAuthoritativeVSyncInterval(ui::Compositor* compositor,
                                      base::TimeDelta interval) override {}
   void SetOutputIsSecure(ui::Compositor* compositor, bool secure) override {}
+  void AddObserver(ui::ContextFactoryObserver* observer) override {}
+  void RemoveObserver(ui::ContextFactoryObserver* observer) override {}
 
   uint32_t next_surface_id_namespace_;
   std::unique_ptr<cc::SingleThreadTaskGraphRunner> task_graph_runner_;