mus: Use the new drag API to implement tab dragging in chrome.

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 d5f3670520cdf5f3aef280027b0d4184d1b8cbd6..3b1bbc4c92b77661e826c815f103373e977104de 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<ui::Window*>& WindowManagerConnection::GetRoots() const {
+  return client_->GetRoots();
+}
+
 WindowManagerConnection::WindowManagerConnection(
     shell::Connector* connector,
     const shell::Identity& identity)