mustash: Enable connections to mus from the Chrome renderer [take 2]

content/common/render_widget_window_tree_client_factory.mojom

Index: content/common/render_widget_window_tree_client_factory.mojom
diff --git a/content/common/render_widget_window_tree_client_factory.mojom b/content/common/render_widget_window_tree_client_factory.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..e6d88d7bfcdbc57a84659d861a856e3d555d8f49
--- /dev/null
+++ b/content/common/render_widget_window_tree_client_factory.mojom
@@ -0,0 +1,13 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module content.mojom;
+
+import "components/mus/public/interfaces/window_tree.mojom";
+
+interface RenderWidgetWindowTreeClientFactory {
+  CreateWindowTreeClientForRenderWidget(
+      uint32 routing_id,
+      mus.mojom.WindowTreeClient& window_tree_client);
+};