exo: Xdg toplevel shell surface can be minimized by default.

We shouldn't require clients to explicitly set that they can be
minimized. The default behavior for toplevel XDG surfaces is that
they can be minimized.

Note: The previous behavior was motivated by the lack of shelf icon
integration, and that making it less obvious how to restore the
window after minimizing it. This is not an issue anymore, with
shelf integration having been added.

Bug: 821498
Test: Caption for xterm in crostini has minimize button by default
Change-Id: I8a730d8e64dc104f990d70b0e147f182cc06e54d
Reviewed-on: https://chromium-review.googlesource.com/1024818
Commit-Queue: David Reveman <reveman@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553093}

components/exo/wayland/server.cc

@@ -1855,6 +1855,7 @@ void xdg_surface_v6_get_toplevel(wl_client* client,
     return;
   }
 
+  shell_surface->SetCanMinimize(true);
   shell_surface->SetEnabled(true);
 
   wl_resource* xdg_toplevel_resource =