mus: Make --use-mus-in-renderer the default for --mash.

content/renderer/mus/renderer_window_tree_client.h

Index: content/renderer/mus/renderer_window_tree_client.h
diff --git a/content/renderer/mus/renderer_window_tree_client.h b/content/renderer/mus/renderer_window_tree_client.h
index 81662e55811ecf84ca031f6cf35ecfe16f1eb03a..9ab9f3f3cc27102f487724e353c0e02f5e0ff8c0 100644
--- a/content/renderer/mus/renderer_window_tree_client.h
+++ b/content/renderer/mus/renderer_window_tree_client.h
@@ -34,6 +34,9 @@ class RendererWindowTreeClient : public ui::mojom::WindowTreeClient {
   // connection to mus is lost, or when the window is closed.
   static void Create(int routing_id);
 
+  // Destroys the client instance, if one exists. Otherwise, does nothing.
+  static void Destroy(int routing_id);
+
   // Returns the RendererWindowTreeClient associated with |routing_id|. Returns
   // nullptr if none exists.
   static RendererWindowTreeClient* Get(int routing_id);