Support additional mash shelf alignments.

mash/wm/root_window_controller.cc

Index: mash/wm/root_window_controller.cc
diff --git a/mash/wm/root_window_controller.cc b/mash/wm/root_window_controller.cc
index d610dc2c3acef594e486fb924d97f8d3b3e95cda..53356d480888f6b1c5434e08ca7fab8f9a6e353f 100644
--- a/mash/wm/root_window_controller.cc
+++ b/mash/wm/root_window_controller.cc
@@ -91,6 +91,12 @@ void RootWindowController::OnAccelerator(uint32_t id, const ui::Event& event) {
   }
 }
 
+ShelfLayout* RootWindowController::GetShelfLayoutManager() {
+  return static_cast<ShelfLayout*>(
+      layout_manager_[GetWindowForContainer(mojom::Container::USER_SHELF)]
+          .get());
+}
+
 RootWindowController::RootWindowController(WindowManagerApplication* app)
     : app_(app), root_(nullptr), window_count_(0) {
   window_manager_.reset(new WindowManager);