Aura-Mus: Implement gutter in the embedder.

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index 6609d5390926a9659bd315cbf384311c78ab7ec9..91fec312c2c0c7455583febc1602244bff1ab347 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -1694,6 +1694,7 @@ void WindowTreeClient::OnAccelerator(uint32_t ack_id,
 void WindowTreeClient::SetFrameDecorationValues(
     ui::mojom::FrameDecorationValuesPtr values) {
   if (window_manager_internal_client_) {
+    normal_client_area_insets_ = values->normal_client_area_insets;
     window_manager_internal_client_->WmSetFrameDecorationValues(
         std::move(values));
   }