Add GetSurfaceManager to ui::ContextFactory.

ash/mus/stub_context_factory.cc

Index: ash/mus/stub_context_factory.cc
diff --git a/ash/mus/stub_context_factory.cc b/ash/mus/stub_context_factory.cc
index 22f5c8eaf851de17ef8e42adf7b3980e967c5392..e07cddb09e132f6f3816b798b991f73800833dc6 100644
--- a/ash/mus/stub_context_factory.cc
+++ b/ash/mus/stub_context_factory.cc
@@ -77,6 +77,11 @@ StubContextFactory::CreateSurfaceIdAllocator() {
       new cc::SurfaceIdAllocator(next_surface_id_namespace_++));
 }
 
+cc::SurfaceManager* StubContextFactory::GetSurfaceManager() {
+  // NOTIMPLEMENTED();
+  return nullptr;
+}
+
 void StubContextFactory::ResizeDisplay(ui::Compositor* compositor,
                                        const gfx::Size& size) {}