Tab dragging as implemented as a mus API.

ui/views/mus/window_manager_connection.cc

Index: ui/views/mus/window_manager_connection.cc
diff --git a/ui/views/mus/window_manager_connection.cc b/ui/views/mus/window_manager_connection.cc
index 4d1fe3bc11b140cd67ac7eff5d48387e1e3ff4cc..70de2e8fc6802054aae46b7744c82f2ba979cbff 100644
--- a/ui/views/mus/window_manager_connection.cc
+++ b/ui/views/mus/window_manager_connection.cc
@@ -100,6 +100,10 @@ void WindowManagerConnection::RemovePointerWatcher(PointerWatcher* watcher) {
   }
 }
 
+const std::set<mus::Window*>& WindowManagerConnection::GetRoots() const {
+  return client_->GetRoots();
+}
+
 WindowManagerConnection::WindowManagerConnection(
     shell::Connector* connector,
     const shell::Identity& identity)